We love our symmetry. I've watched with fascination as people line up tableware, straighten wall mounted pictures, even get plastic surgery in the name of perfection.
When it comes to setting up a stereo system symmetry gets us in trouble. This is a really hard one to battle because our tendency—my tendency—is to whip out the tape measure and get the speakers symmetrical within fractions of an inch. This is great for positioning the center chair, but not good when it comes to speaker placement.
In the same way non-parallel walls are beneficial to the elimination of standing waves, placing the speakers slightly off perfect often solves big imaging and bass problems.
So, the next time you find yourself stressing over laser-accurate speaker placement, remember to slide the whole affair slightly to one side or the other.
Though it rubs against the grain, a little asymmetry goes a long way towards sonic perfection.