Like most things, It's a matter of taste. I have a project studio where I have an LEDE (live end dead end) setup where the entire third of the studio with the monitors (front wall, ceiling, & side walls) is covered with absorptive material (and carpet on the foor) and the back has diffusion. This minimizes the effect of early reflections and allows you to hear 'into' the room where the recording was made. Imagining is amazing with pinpoint accuracy but it has a character unlike any other listening room. In my music listening room (where I listen to classical music) I have two 2'x4' absorbtive panels mounted horizontally behind the speakers with about .5' between them - works for me. Try different amounts in different locations (as well as diffusion) and see what you like best.
