Well always remember, whatever you do on the recording side stays on disk. If you run too much compression while you're tracking, you're stuck w/ it, complression isn't easily undone....
If you wait until the mixing side, you can mess around w/ what you like and still make changes. If you only have one channel of compression, then you would have to run each track you'd like compressed, through the compressor individually.
Not all tracks need compression either. Depends on style you're going for , equipment used, and mic techniques/placement.
Don't overuse the nose gate either. Bleeding from mics isn't the devil...poeple get caught up w/ just getting the source sound and forget that you can lose a lot of the feel of the mix by gating everything to death. It's okay to have a little snare sound in your tom mics, or a little guitar in your overheads.