Using an equalizer (including the built-in auto-equalizing that a lot of receivers have these days) can help mitigate tonal differences in speakers. Because I don't have a lot of experience with different speakers, I still lean towards the thought that the front and center speakers should be tonally matched because differences in them will be pretty obvious when watching movies/television in surround sound. The surround speakers aren't as critical, IMO. There isn't as much audio going to them, and it won't be as obvious if they aren't tonally matched (and the EQing can help to mitigate it). Using vastly inferior speakers in the rear will probably be obvious to you, though.