I don't want to pretend I have knowledge of design characteristics - I don't. My opinions are based on accumlated reading and one small personal example. And that example is for a short while I had one M22 about 2 feet higher than the other. During that time the "sound field" was noticeably distorted. If I had a right and left even with my screen and a center offset by the amount necessary to get it above or below the monitor, I think the sound would appear equally as distorted. Using the horizontal brings the offest more into alignment. And I think all that really ties back to your qualification of possible using a horizontal speaker due to vertical space limitations; if I were using a screen, I'd definitely get the three matched.
Now the OP seems as if he's putting them all on the floor, so no problems there. He might want to consider stands that tilt the speaker up a few degrees.
I hope I'm not coming off argumentative here, I'm enjoying the discussion and picking up more info.
