It's a good idea to keep the output channel clean while mixing. Don't put a compressor on the bus because you'll have to take it off for the mastering stage, and if you mixed with it on, your mix will get all screwed up when you take it off. Also a Logic compressor on the master bus does not sound good at all.
Best bet is to resist temptation and not touch the master bus until you've finished your mix. Then bounce it down and work with the two channel stereo file(s).
When i do rough "mastering" jobs for non critical demos, etc. I use a combination of izotope Ozone and PSP Vintage warmer. I don't consider myself a mastering engineer by any means, but i can get a very good sound out of these two, mainly just because i've been playing around with them for a while now. These two plugins tend be very heavy on the CPU too, so i'd only really use them on in a two channel situation.