There are somewhere between 5 and 50 new versions of rockbox *every day* - we do a new build every time any code is changed. Update as often or as infrequently as you feel you need to. The exact details of the changes between revisions is here:
http://www.rockbox.org/recent.shtml#svnbut this information is not really targeted at end users.
Updating Rockbox is supposed to be extremely painless; you just unzip the new build over the old one, and all settings/etc should be preserved. The Clip is still an in-development target, so you may find there are fixes/improvements for it fairly often.