I have to agree on the VBR encoding
average mp3/musix consumers are happy enough with the 128kb stuff. But for the pro's (even for us eh!) it's far from any-good.
VBR (Variable Bit Rate) encoding takes LESS bytes but gives BETTER quility than average mp3 encoding.
although not all players support the VBR format... winamp does!
I use forced stereo by the way.
but for the real players here, I recommend MP+ !!!!!!!
did some serious tests, and MP+ was the overal winner. period.
But to get back to the original topic: I think YES you can judge a great deal on mixing using mp3. We here at the board are smart enough to hear what's going on.
(even at 22kHz/64kB) only the Mastering process is difficult (when using low bitrates) to judge.