Yes the only way to ensure a perfect match of left main, center, and right main is to use the exact same speaker in all 3 positions. I believe there are a few other reasons he prefers a vertical center, but I can't remember them at the moment.

I use a horizontal center and am completely happy with it. Would I claim it is a perfect match to my left and right mains? Nope. Do I notice any difference between it and my left and right mains? Nope!

Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton