Anything in /System/Library/Fonts/: don't touch
Anything in /Library/Fonts/: don't touch
Anything in ~/Library/Fonts/: touch
Not true. I have pulled everything from /Library/Fonts/ except the basic MS set (verdana, etc). Things work fine--even iCal without Neue Helvetica.
You could begin to pull fonts from /System/Library/Fonts/ on a trial and error basis. Japanese fonts could go if they aren't crucial to your work. Proceed at your own risk and test rigorously.