Most everyone is going to tell that you should only compress after tracking because if you compress to much while tracking then your stuck with it......And since you don't really know how much compression is needed on each specified track until after the tracking is done, then it's best to just wait.....That's what everyone usually says
I on the other hand compress while tracking alot....It varies though, depending on the type of tunes I'm recording and the instrument being tracked and the skill of the musician who is playing the instrument.
The easy way to compress after mixing is to use software plugins...You set the ration press a button and ba da bing the track or the entire mix can be compressed *don't try and compress the entire mix all at once unless you know what your doing, or at least that's what they tell me * ......You could also run the entire mix back out of the tracking device and into an outboard compressor and compress after tracking that way........It's your choice!
hope that helps
- nave