New jars or used jars? I stopped reusing jars after I had a few blow-out during the waterbath. It
didn't happen with every jar but when folks remove the contents (usually some kind of preserves) they tend to scrape the bottom of the jar with something metal like a spoon or knife.
The scraping creates micro-scratches which effectively score the base of the jar. Under the pressure of a water bath, the scoring can cause the base to separate from the body, emptying the jar's contents into your pot.
Not only do you lose a jar but the mess is a pain to clean. You also have to wipe off and clean all the other jars which, because directions usually call for the jars to be allowed to sit over-night, can be counter productive.
After losing 3 jars out of a 12 jar batch I decided it was more than I was willing to risk. These days, I only use brand-new jars and lids.