There may not be a rule of form about about placing a television over a mantle, but functionally there are very good reasons not to do so. Any good professional installer or amateur videophile will tell you that when installed, the center off the screen should be at or near the viewers eye level when seated. Doing so greatly reduces eye strain by keeping you eyes in a more or less relaxed position.

In this instance, function should take precedence over form. From a functional standpoint, placing a TV over a mantle is one of the worst widely accepted decorating ideas to come down the pike in quite some time.