Practically speaking most people will put below the TV because u tend to have more support (e.g. a shelf or cupboard below the TV). If you put above the TV you need to install a mount, and it's not easy finding a solid wall mount for center speaker (which can be quite big).

Aesthetically having the center speaker below the TV makes it blend in more with the rest of the furniture. You put it above, it's gonna stick out (like a sore thumb).