If I canned tomato sauce and all the jars sealed perfectly, can I open them, cook the sauce a little more, and do the process over again?
I had some of the sauce today and I think it needed a little more cooking.
We make a lot of sauces every year. I wouldn't try to do it over. If you know what its lacking just add it to your sauce as you use it. Sometimes bland sauce can be a good thing and offers you an open door to add what you want. If you're missing some flavors you like or don't have enough of it just add it for now with seasonings. Make a mental or written note for next year to try and zero in on what exactly you want to produce.
Basically, every time you open the jars you are exposing the food to the risk of contamination. If you are planning on storing the sauce for only a few weeks it doesn't really matter, but if you plan on storing it for several months its important to heat the sauce on a high heat to make sure nothing can live there. Even more important is to clean and sterilize the jars before re-sealing them with the new sauce.