It's normally up you, the user, how often you want to update. But speaking as a Rockbox tester, I usually:
1) Update when there are major changes
2) Find an issue with any SVN build I'm using at the time. and need to verify if it's still carrying
3) Need to report problems as per the forum and IRC guidelines
I would recommend trying to keep yourself as up to speed as possible. Rockbox changes every day, and bugs that may crop up in one build may get squashed within a few builds.
Also note: No features will ever just get pulled, unless there's a damn good reason for it. So sail worry-free, knowing that you're a part of a great community here.