Why? If you're not hitting your head, it doesn't matter if you've got 1 foot or 10 feet of headroom.

One thing I think though, is that momentary clipping is more common than people realize, and that the recovery from a clip varies from amp design to design.

In that case though it's not headroom, but used power.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris