I had a long, rambling discussion about this subject on another forum. The chap I discussed it with was fairly knowledgeable about this kinda stuff, and I took his advice in good faith as he wasn't trying to sell me anything!
His point was that a note (or other sound) should sound the same whether played through the front or rear channels, and the only real way to be sure of that is to use identical speakers. Makes sense I s'pose!