IMHO if your tank is against a wall the only purpose of a backing would be to hide the wires and junk behind the tank. If you wanted to limit light in an effort to reduce fish stress, you could cover three sides of the tank and limit/eliminate excessive motion in front of the tank. I don't feel that a back is all that important. One of my tanks has a back (blue posterboard) purely for my own sense of aesthetics the other has nothing but the wall.
\Dan