Robb, the point is to have the sound from the mains and the center reach the listener at the same instant. If this is done from positioning alone, the center speaker should actually be slightly farther back in order to be the same distance as the mains, but one doesn't have to rely on positioning alone; the receiver has distance settings for setting up the speakers which can adjust for a center speaker which is closer by delaying it slightly. So, this isn't a reason not to have the center speaker slightly overhanging the edge of whatever supports it.
Again, my preference is for the wider and smoother horizontal dispersion of a vertical center speaker, as compared with most(all?)horizontal configurations. I'd use M22s all across the front and put the rack wherever else was convenient.
