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It seems the usual procedure is to sanitize cans and lids by immersing them in boiling water, to fill the cans, and then to cap them and boil for a few more minutes.
If I sanitize a can and then fill it with boiling jam, and cap it with a sanitized lid, then why bother with the extra boiling? It seems like a totally redundant step.
Conversely, if you are going to immerse the filled cans in boiling water to process them, then why bother pre-sanitizing them?
This makes no sense!
These various boilings serve different purposes.
The jam mixture is boiled to reduce the liquid and bring the pectin to its gelling temperature.
The bottles and lids are boiled to sterilize them as you said.
The final water bath kills any bugs that were introduced while filling the jars. Additionally this boiling ensures a good seal on the bottles. In the time it takes to fill the bottles the jam will have cooled somewhat and you might not get a good seal.
Killing bugs with boiling is not an instantaneous process. It takes time at those high temperatures to kill a high percentage. If the bottles aren't pre-sterilized then you are putting yourself at a disadvantage as those bugs would be distributed through the hot, but no longer boiling, jam.
Of course, all of these boilings will do some sterilization and are somewhat redundant. It's all about reducing risk. You can bottle with clean but unsterilized bottles and if you work quickly you can skip the hot water bath. Doing so will greatly increase the chances that one or two bottles might not seal or that some bug might survive and spoil your stuff. Reliable sources indicate that if bottles are not presterilized that the bottles should be processed longer in the water bath.
Anecdotally speaking- I have in the past skipped the water bath when my jam was very hot and acidic. I would usually have one or two bottles out of 20 that wouldn't seal. I no longer skip the water bath and I almost never have any bottles that fail to seal.
I have never had jams go bad but this is more luck than proof.
My answer supplements Sobachatina's.
It's a matter of risk, and the issue is that what you're risking is a horrible death.
If you merely wash the cans, fill them with a hot mixture, and then put them in a hot water bath (or pressure canner), then you are probably killing 95% of the bacteria and fungi that you'd have killed if you also sterilized the cans. The problem becomes that if what you're missing is even a single botulism spore, then you can produce a can which, for days or weeks, looks OK, tastes OK, and will kill you.
So it's a matter of how much risk you're willing to tolerate. What you're canning also makes a significant difference; high-acid or high-salt foods (jam is generally high-acid) are significantly lower risk than, say, canned tomatoes.
If you are going to skip a step, sterilizing the cans is the step to skip, rather than the water bath. The reason is that the air in the can has bacteria and fungi in it, so you need to sterilize after filling the can (or jar).
With jam, I make my jam while sterilising the jars in the oven. Then I put the jam in the very hot jars and put the plastic cellophane-like jam covers on. The jam lasts for months, even years, even after opened, without going mouldy, plus no one has died yet. Sometimes I put the pop-down lids on like comes on jars when you buy them, or I use preserving lids if I've run out of everything else. Jam is high in sugar which is a preservative (I believe pectin is also, as well as being what sets it). Low-sugar jams last less time, and may even need to be refrigerated right from the start. Other types of perserves need other processes - low acid, low sugar foods need to be pressure cooked. Foods with either high acid or high sugar can be done in a water bath or in the oven.
So, to answer your question, I would say no, it isn't necessary for jam, as neither me nor my Mum nor anyone else I know has done it and we are all well and have made lots of jam!
I've been using the inversion method for many years. But when giving them as gifts I tell them to use it right away, and any jars with a failed seal go in the garbage.
I make fabulous combinations of jams (peach - raspberry - habanero for example). I do boil & sterilize jars prior to filling as well as the lids. As I'm filling jars, if any seem suspicious it goes in the fridge and gets used first. I keep them in my pantry and check lids regularly, and throw away any that fail. I believe that we need to use our good judgment. Do research but don't be overly paranoid.
There is a canning method for jam which skips the second boiling. It's called the inversion method. Basically, you pour the hot jam mixture into your jars, add lids, then immediately invert the jars. The boiling-hot jam coats the headspace and inner surface of the lid, sterilizing them and the air bubble.
Edit: The inversion method is currently not recommended by the USDA (PDF).
Any food product with a pH higher (less acidic) than 4.0 must be pressure canned to guard against botulism, which can be odorless, tasteless and deadly. Botulism can grow at pH 4.6. I bought pH strips (they're very cheap insurance) and test each batch of food prior to bottling.
Foods that are sufficiently high acid (pH 4.0 or less) and therefore not a botulism risk I boil, pour boiling product into sterile jars, top with sterile lids and rings and allow to cool - yes, this is skipping the water bath step and increases the odds that airborne bacteria/mold/fungi might successfully grow inside the sealed jar. Personally, I've never had a jar go bad and I've been canning for about 40 years. If a jar does not seal, I refrigerate it for prompt use or water-bath-process it so that it seals.
If a contaminated high-acid product were eaten, it could make the consumer sick. Which is why, whenever you open a sealed jar of home-preserved food (regardless of method) you must inspect for intact seal and food quality. If you notice any off colors, odors, or see evidence of mold, toss the food without tasting it.
I am not an expert; I have completed a Master Food Preserver course.
We make Apple Butter and Salsa in the manner you described.
The Apple Butter is cooked for approximately 12 hours in a slow cooker (lid on) at a temperature varying between 280 to 325 degrees. We stir occasionally and use a hand-held blender to smooth it.
We boil the jars and lids and our handling instruments at a full rolling boil for 15 minutes. I make certain that Apple Butter is just at the boiling point when canning and that's around 250 degrees. We work hot on hot.
We put the Apple Butter in the jars, put on the flat lid, then the other lid and wait. 95 percent will seal. If not, we use right away.
Now, a couple years ago we used to just use the dishwasher to sanitize. (My Dad and Husband like to take shortcuts). I insisted on boiling! The difference is this: the Apple Butter has a LONGER shelf-life once you've opened it if you boil the jars. If not, it spoils faster in the fridge. The difference is like a month, or more! I was really surprised at the difference.
We do the same with Salsa. We add a large amount of vinegar and lime juice to take down the acidity. The Salsa cooks at around 300 degrees for about 2 hours in the same cooker and again I make sure it's HOT when putting in the jars. Hot on hot! Again, the shelf-life once opened is longer than if I'd just sanitized in the dishwasher.
Now, this technique is predicated upon sterilizing EVERYTHING. Make sure your hands are squeaky clean or wear gloves. Make sure counters are spotless, etc... I would boil the jars again were I not dealing with such hot canning mixtures.
Were it anything else I'd do the double-boil technique. Low acidity products, raw stuff, meats, a cool mixture etc... and if you're going that far, a pressure cooker is not a bad idea.
Anyway, I think canning is fun and I'm going to try some new adventures next summer using the double-boil technique and the pressure cooker.
