If you're wall mounting it, it should be as low as possible while clearing any furniture or fixtures. Mounting low is even more important if you're mounting it on a table or stand (recalling the recent rash of children smacked by falling televisions).
Eye level is about 40" and how close you need to be to that number varies with how wide the vertical viewing angle of your TV is.
Of course this eye level should strike the center (or slightly above) of the screen.
If you're bound and determined to mount the TV up high, you'll need to put a rake (noun, inclination from perpendicular or horizontal) to the TV such that the screen is still perpendicular to everyone's line of sight.
See Mistake #3 in the following commentary:http://www.televisio...kes-1-to-10.php