Honestly, has anyone else used 19s & 20s? I do, and I don't think they hurt any more or less than a 22 (which I usually use when there are plenty on hand). I have used many 19s, 18s, and even a 14 (on a dare), and didn't have a problem. I use what's available. The only difference is that you do get a few more bleeders with the heavier gauge, but that's not a big deal either, IMHO.
The other issue is that 19s are usually the easiest to get, as they're the standard for all horse injections, which are mostly over-the-counter. Many human injections also call for a 19, including antibiotics.
Charles