If the speakers are the same model and positioned similarly in relation to the listener, this works quite well.

That's kind of what I was getting at I think. That makes sense.

Still, for some reason my mind is trying to convince me that the center channel needs to be "horizontal" for wide dispersion to mix properly with the left and right mains properly. In typing that, I realize it's false. But the idea of multiple bookshelf speakers for centers is a very new notion for me.

I definitely believe you. I should strongly consider a pair of m22s for centers.
2 Axiom M80s v3-custom
1 Axiom VP160 v3-custom
2 Axiom QS10HP
2 Cerwin Vega CMX 12's
Pioneer SC-57