Yeah, always seperate the two. Never try to master while you're mixing. All though I have heard of many pro engineers mixing with a compressor on the main buss...but not a full-on mastering chain.
I also seperate tracking from mixing, I used to find that when I was tracking my own stuff I'd start dicking around with the mix and not ever get anything done. I'll track all the instruments and do very rough mixes with absolutely no effects, comps, or anything. Then I'll start from scratch with all the tracks and do the final mix where I apply comps, EQs, and effects. I'll then master the stereo track.
Your mastering effects, if applied right, should affect the mix very minimally...or at least in my view. I first use a parametric EQ with very gentle cuts and boosts to get the song sounding right. I'll then use a C4 compressor or another multiband compressor to boost the low end a bit and to keep the peaks calm so the loudness maximizer isn't going crazy. The key here with using the multiband on your stereo mix is the attack and release times...those can REALLY alter your mix, so be careful not to bury your drums with attack times that are too fast...just try to keep some of the larger peaks under control. I'm in agreement that 2:1 is a good ratio for the multiband on the stereo mix.
Ultimately if all you really want is hot mixes, you should really use limiters or comps with heavy ratios during mixdown...don't just depend on your loudness maximizer plugs during mastering.