All I can relate is my own experience with my M60s. In short, they seem to be very forgiving and I just can't seem to make them sound bad.
I have experimented with my M60s with just 4 inches of clearance to the back wall and then out over 1 foot. I personally could not tell the difference in my room. This could be very room dependent though and I don't have a trained ear.
I am also sometimes forced to put my left front in a corner to avoid blocking a room I only sometimes use. 95% of the time, I pull it in closer to the center to match the other side. I seem to notice a difference but I can't really say it sounds worse, mostly my sound stage isn't as good. I can't be sure if its because I've moved it into a corner or because they are now too far apart.
Nothing conclusive for you there but hope it helps.
