jeez guys
think about it this way... if you put your head close to the monitors, you notice big time the difference in balance as you move your head up and down (with the speakers places as they should be, vertically). if you place them sideways, anytime you move left/right you lose your sweet spot. also, you get wierder phase/lr placement because the speakers' relative distance to each other (not positive about that, but i think that's what i remember, I'm positive about the vertical placement, tho). you want your 'sweet spot' i.e. the place where you keep your head most of the time, to be in about an equalateral triangle, with your ears at about mid baffle level. this will give you a more consitant sound for your mix as you move about changing things, etc etc....
also, another REALLY UNDERSTIMATED factor in monitoring is placement on top of/in front of things... early reflections can REALLY fuck up your mix, causing comb filtering which CANNOT be compensated for with EQ (comb filtering is where the short/midrange frequences are phase cancelled out by the waves that bounce up from those nasty early reflections....
hope that helped.