WELL,
setting up a mastered 'pro' mix to a/b against,
is still the very best approach...
where most people drop the ball,
is in not understanding how 'mastering' is going to effect your mix,
and how you have to mix with the knowledge that the final stereo mix is going to get mastered.
what happens to your kick or snare levels once mastering compression and limiting is applied?
what happens to your reverb levels when the overall level of the mix is brought up to proper mastered levels?
what happens to the sibilence in the background vocals when mastering eq is applied?
if you go blindly into these things, your end result will not match your expectations.