agree with retro...there's the center...and then there's the 'center'. Back in simpler days, one local pro strung his PDP Fiberstaff with 8 mains and 8 crosses intersecting in the 'center' of the face. He would hit a bit and teach a bit with that set-up to demonstrate the 'reward' from keeping ones eye on the ball...( and to suggest to those who thought they needed that newfangled Prince behemoth that 'less is more'
For most wood rackets, the hitting zone was low of center, especially as a racket wore out a bit. One advantage to stiffer metals and composites was the extra strength of the upper hoop, allowing better response from balls hit higher in the face.
THEN, you had guys like me who hit 2-3 mains off-center because of the amount of topspin we were trying to create(before EVERYbody started playing that way
) . Still needed to hit close to the middle, but I'd have to string that practice racket non-symmetrically and keep a consistent grip orientation.