Iv'e read the Metagross analysis, and it doesn't answer this qiestion, so: should Metagross use Zen Headbutt, or it has bad coverage?
Psychic moves generally have bad coverage so using them if you have another STAB is generally not worth it. The only real use for it, IMO, is 2HKOing defensive Rotom-A with a Choice Band. Even at that, being locked into a Psychic move lets powerful Pokemon like Tyranitar and Metagross switch in for free.
Edit: Evil Ragoo, I think this thread is what you want.
http://www.smogon.com/forums/showthread.php?t=67702