My room is 16' wide, with a 10' high ceiling. Soffit and light shelf take up two feet, so the side/back walls are 8' high.
The middle 11' of the back wall, 8' high, will be treated with 2" 703. The angled entrance area near the door won't be, and there is a closet door on the rear right that won't be. See my sig for a link to pictures that may help understand.
One problem I can see with not treating the back wall: It is a first reflection point! My rear seat ears are only 3-4' from the back wall. Back wall reflections will likely be even closer in time than side wall reflections for the back row.
We'll see how it goes.