Graeme - If you've got the extra speakers, I don't think there is a good reason not to use them. I suspect, however, that the added benefit will be fairly minimal (compared to the difference of adding 2 surround speakers to a 3.1 system)

Personally, if I had the speakers, I would probably use them unless there are other non-audio reasons not to (i.e. they don't look good mounted on the rear wall)

Jason

Epic 80-800: HG Cherry