okay now I will open a large can of worms..... (proceed carefully)
We generally use the modified LEDE approach called RFZ (Reflection Free Zone) that is intended to control and or steer all early reflections from the listening position... This calls for the rear wall diffusor installed with the guidelines already listed in several other posts... (time and distance).
I do make use of Abfussors in the front/side walls in a lot of our designs and these are often mistaken for diffusors. They are QRD based absorbers from RPG that really clean up imaging issues for us very well when placed at positions of first reflection. They look like a QRD, but are made from Homosote and Rigid Fiberglasss (absorptive materials). They provide extended LF absorption for their 4" depth by the nature of the phase grating affect....
We DO use front loaded diffusors in dedicated surround suites. I always thought that for a true 5.1 installation the surrounds should get "equal consideration" as far as the diffuse return. Placement is tricky as you don't want these diffusors to be "visible" to the direct or first reflected sound from the front speaker array... This is where we earn our money!!
But generally, for a stereo critical listening environment, we do not use front diffusors. We don't want diffuse return in that short time frame as it will cloud the imaging reproduction in the recording... (this is another large can of worms)...
I think I'll get Lars to post his paper he wrote on the implications and affect of early reflections in critical sound reproduction. It uses the TEF ETC displays to graphical demonstrate what happens with a lot of early reflections in a playback environment. Good stuff!!! Lars, if you read this, please consider posting that paper online here.....