Can Kelsley Grammer and David Hyde Pierce aka Frasier and Niles Crane really play the piano in real life?

Kelsey Grammer can - it's in his autobiography. He was expelled from Juilliard where he was mainly studying Drama but did a bit of music too. I have no idea about DHP but he looks like he can (ie when you see him 'play' he doesn't look as clueless as most people faking it do).

Yes, DHP plays both piano and organ.

David Hyde Pierce is an accomplished classical pianist, and studied piano many years at Yale, afterwhich he turned toward acting. Kelsey Grammar was studying acting at Julliard and learned piano on the side, with his focus on jazz piano style. Kelsey was expelled from Julliard (unknown reason).
I recognized right off that both Niles and Frasier could play piano fairly well, as in the scenes the camera angles weren't the "usual" way of filming to make it look like they were playing when the actor couldn't really play (if you remember how they filmed piano scenes of Charlie Sheen in 2.5 Men)
source, IMBd "Frasier" http://www.imdb.com/title/tt0106004/