Are you starting a new forum or upgrading a previous forum?
If you are starting new, I wouldn't hesitate to start with RC4. I have been running phpbb3 since RC1 and it's been nothing but stable. Mine is a small board (45 users, 3000 posts) and everyone likes it better than 2.0.22 that I was running before.
If you are upgrading, then it can be a bit more difficult if you have any mods installed. If not, then again, I wouldn't hesitate in upgrading to RC4. As you will find in all posts with regards to upgrading, make sure you have backed up everything (into 2 separate spaces if you're as OC as I) before you begin. It's not hard to upgrade, but just in case something beyond your control happens (power spike, hd failure) you'll need those backups. If you have a heavily mod'd 2.0.22 board then upgrading to phpbb3 is going to be hairy no matter if you do it now or wait until gold.
If you make style changes, create your own style, that way when you upgrade anything changed in the default style won't interfere/overwrite your style.