Yes and no. You always need absorption in corners for bass traps, and I prefer absorbers at the side-wall and ceiling reflection points. Then you can use either absorbers or diffusors elsewhere. Most people I help in forums are on a budget, and good absorption costs a lot less than good diffusion. But when having the "best" matters more than cost, diffusors all around a room - and especially on the rear wall - can be awesome. In case you missed it, this recent video lets you hear the sound of a room with and without extensive diffusion:
Hearing is Believing
--Ethan