Del
Also mentioned here is another very good point: and that is NOT being setup in a corner of a room as a listening or mixing position! I ran that way for years until I read somewhere (sorry I can't remember where) where that is a BIG, BIG ....NO, NO as to where you want to be listening to your music.
All other things being equal (which they're not), facing a corner - with monitors in their proper positions relative to your listening position - solves the problem with (higher frequency) side reflections as they'll bounce around quite a bit before getting back to you. You can also fill the entire corner between the monitors and thereby deaden the front end and acheive an enourmous base trap. You still have to worry about the ceiling and the fact that response will not be symetrical between sides (it often isn't anyway, depending on the room).
My daughter's bedroom monitors are setup this way in a corner (mostly for practical reasons) but without any treatment for the moment.