This is not a "best practice" anymore in design. Mostly because people started developing neck injuries from sitting with their heads tilted back for hours on a weekly basis. If you can avoid mounting it over the fireplace, then you should.

If there is nowhere else in the room to place the TV, then you should install it on a drop-down arm that allows you to lower it to viewing level when used in conjunction with seating. At that point, the height isn't really an issue.

The center of the screen should be at eye level when you are seated so above a fireplace is never the best place .