Anyway, how do you know where you are supposed to put the panels? Do you get a consultation first, or just put em on whatever walls you feel like?
Either way. Well, not so much "feels" like, but you can experiment, and add to taste. OTOH, you may perceive little to no difference if you only put in a small handful of panels, and/or because the panels aren't very thick/effective enough for the application.
The easiest way, I think, is to post pics, attach them. Better yet, you can directly ask companies like GIK or Real Traps. Their advice is free. Pics will still be very helpful.
My guess as to where it's hardest to go wrong in treating are front wall corners for bass, and perhaps directly behind speakers to reduce SBIR, but even the latter may not matter hugely; it's dependent on how close your speakers are to front wall. Or, for people who are up against the back wall, then that wall becomes a prime candidate. As for sidewalls, it depends on who you ask, as well as specifically the offaxis performance of your speakers. (If they are superb in that manner, then the sidewall reflection still is very close to the same response as the onaxis response; but, I've read that this may be more pertinent to stereo, and less the case with mch, but not exactly sure . . .)