There are still a lot more Red Hat 6.2 (Open)ACS servers in production
than Red Hat 7.1. So in terms of "what is tried and trusted", the answer is probably still 6.2.
Having said that, my test box is now RH 7.1 and I have not seen any
7.1-specific issues that worry me regarding RH 7.1 and OpenACS 3.2.5 or 4.x.
Some folks feel that the Linux kernel 2.4.x series (as used by Red Hat 7.x) still has some VM issues to resolve before it should be recommended for production servers with desired uptimes of a few (tens of?) months... I've not stressed the VM subsystem of a 2.4.x kernel enough to offer a valid opinion on that one!
Personally, I'm starting to develop and test my RPMs on 7.1, and then backport to RH 6.2, rather than the other way around... and as long as I have good access to machines running both 6.2 and 7.1 myself, I'll be about equally able to "unofficially support" OpenACS folks using either of them.
My one concern with recommending Red Hat 6.2 for new server use right now, would be that Red Hat is likely to stop producing updated RPMs for it once RH 8.0 comes out in around April 2002, if history is any guide. So using Red Hat 7.1 might be good from that perspective.