It depends on how big the room is.
I have a Sony 5.1, 100W/ch and sometimes I turn it very high, but at some point it just feels like a loud noise thanks to the smallish speakers. I know I could go louder with bigger speakers, but probably wouldn't hit the max.
That being said 100W/ch is plenty for an average room IMHO.
I know someone who has a 2x500 tube amp with nice big speakers and he can turn it up to the clipping level without making it sound too loud.