if i understand what ur saying
you want your buss/group to be stereo
because your gtr tracks are panned to the L and R
if you were to make a mono bussgroup it would take your panning options away
the only reason you need mono outs for let say you are using an external mixing board to mix and you are using your daw just as a multitrack recorder, you then send everthing out its own output.....but if you are mixing ITB the yea you should be using a stereo output bus and if you have multiple outs on your interface you can then use those and sends or cue sends for different head phone mixes