Everyone has their experiences. I found that controlling the 2-foot reflections to the rear is the number one key to great imaging. But how? In my last HT room, a small guest room, I used 4 ' x 7' x 2" OC boards behind the mains. I did not treat to the sides. The soundstage went from vague left-right-center to wider than the room. Really a double take when a sound came from outside the room! Side reflections were about 3 feet away. Experimenting, when the mains were within 2 feet of the sides, the imaging went fuzzy. Lesson is to experiment. Your room, your speakers, your preference. There is no right or wrong answer.
My current HT room is about as bad as one could design. I am forced to put my mains almost 8 feet up. ( 9 foot ceiling) They are within 2 feet of the sides. Imaging is terrible and unless a solid center, it is decidedly not from the screen. I got the response better by converting from boxed to in-wall plate using the same drivers ( 4 crossovers later). Brainstorming how to do better, but with doors where I want the speakers, not much I can do. One possibility is auxiliary tweeters moved down to the upper corners of the screen. Might make it worse, might make it better. Another thought was to use line arrays of 2 inch drivers in the room corners in a triangular enclosure so it does not intrude on the door frames. Might get some corner horn argumentation. I would need to find the old papers on how to wire arrays in order to control the phase relationship. There is some trick mixing series and parallel connections without using all-pass filters. In my old living room, I had the space for the mains to be away from everything so I had no dedicated treatment and a very decent soundstage.
I say this as the lesson you already know, but so many miss. The room is half of the speaker system. What works is a unified approach.
