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.