I thought about this a lot in the design of my home theater.
The screen is at optimal height which is with the bottom of the TV 30" above the floor. The tweeters of the mains are 36" above the floor and so 6" above the bottom of the TV.
So at first you would think the having the center below the TV would be optimal.
However the heads of people talking are almost always towards the top of the screen. The center coaxial driver is right above the top of the TV, and the speaker's front panel angles the drivers over the listening area. Room coverage is much better with the speaker above the screen rather than below.
I'm lucky in that the sound never localizes to an individual speaker. The fact that the center is above the TV pulls up the sound stage to pretty much the center of the TV. For center dialog the speech seems to be coming right out of the mouths pf the actors/presenters.
So I have to admit to a strong bias for centers to be above the TV, especially in bigger rooms.