I've done both - quite a few times. I did hit a small problem on one server, but that was with MySQL4.1. If you're running MySQL4 or 4.1 be wary, otherwise I'd say you'll be fine.
I think I've done upgrades from 7.1.3, 7.1.4, 7.1.5, 7.1.6, 7.5.0, 7.5.1, and from 6.0.x -> 7.0.x -> 7.1.6 -> 7.5.x. Basically I've done most combinations, and not experienced any major problems except as above - all RH9/FC1/FC2/RHEL3.