The "theory" is that direct sound is dominant. If you change the listener position w.r.t. the speakers, than you also need to change the delay timings for the speaker which the processor does as well as the levels (and FR to some degree). So whether you mix with one speaker layout or another, unless you room is poor, if you are following the Dolby setup requirements, the sound should be very similar as frequency response, level and time of arrival will be the same in both instances. The variable is the room / speaker interaction, and in a mixing room that must be controlled.
@thespeakerdude I realize this but does it make sense to mix at one position and listen at another? Just thought that was an interesting thought, makes me think that so many things we do in sound has a coating of silly mythology in it.