My powder blue grazes, but not a crazy amount. It is pretty aggressive, especially to new fish. He chased all of my fish for a while. Now he only chases my tusk and sometimes my naso, which is quite a bit larger than him. The naso never grazes.
I've never owned one, but I've watched a lot of Koles. It seems like they are always grazing and they have the mouth shape that makes them well-equipt to do it. Powder blues have small mouths that don't really open and conform to the rocks like a kole.