In my experience OS X starts fragmenting files when you get down below about 15%. So I try and keep 20% free on my Macs.
If you don't deal with too many large files you can likely get away with a bit less free space or you can occasionally defrag with a utility like iDefrag.
A few ways you might consider saving space:
a) Purchase Xslimmer and SLIM all your applications. Not only will it save you quite a bit of disk space but most apps will load noticeably faster.
b) Download SmartSleep and have it just sleep rather than smartsleep. It can delete your smartsleep file then saving you about 2gb of disk space. Just don't let you battery run out while sleeping or you could loose any unsaved work.
c) Reinstall OS X from scratch using Options to NOT install any applications, drivers, etc you do not use. When Snow Leopard comes out you could save considerably more as its base install is a lot smaller.
Have fun.