Flutter Echo -- small room

I've been doomed by flutter echo in a small square room and after months of trying different treatments, I've finally killed the problem... or wounded it anyway.

The question is: Do you have to treat every square inch of one of the two parallel walls facing each other? Or just the area directly to the side, front and back of the speakers.

If the sound is coming from the speakers, it would seem to me that the only angle it could flutter back and forth are those areas coming directly from the speakers, yet I keep reading how you have to treat the whole wall?