How much free space should there be on the start volume?

According to this About.com page written by Tom Nelson:

I would say keep at least 15% of your startup drive free at all times...so that some basic OS X maintenance scripts will have sufficient free drive space to run. This includes OS X's built-in disk defragmentation system, memory swap space, and enough space to create cache and temp files when OS X starts up, while still leaving room for basic applications, such as email and web browsers, to use free space as needed.

I've run for 3 years with no more than 10GB ever free on my 160GB disk, and quite frequently as low as 500MB free. Things seem to work just fine.

Aperture complains loudly if there's less than about 1GB free: I think that would be a sensible minimum to stick to.

Your system swap file does get recovered without a reboot -- if memory usage drops enough.

I find that Caches, particularly those of Safari.app and Spotify.app are a big cause of unnecessary disk use. You can rm -Rf ~/Library/Caches whilst the system is running without any bad effects if you're about to run out of space!