Take a look at this article, especially the last comment. All identical speakers is the absolute best for a multi-channel system, theater or audio, but difficult to use without a dedicated room. The best alternative is a timbre matched system using center and surrounds designed for the mains.
The rears and rear surrounds are a purely "manufactured" "fill-in" signal path sourced from the pre/processor. It's not the same as creating a seamless soundstage across the LF and RF as in two channel critical listening.This was true many years ago with Dolby Pro-Logic, however, today all channels are discrete, uncompressed, and full bandwidth. A sound may move through the room in anyway imaginable, or be fixed at any place in the room, therefore any two or more speakers may be used to create the sound. Looking at the picture in Matching Front and Surround Speakers, it is obvious that the jet would sound exactly the same, front or rear in the room, with all identical speakers. Using timbre matched rears should be extremely close, but still not exactly the same, however, mismatched rears will have a definite difference in the sound. This is easily heard by running the test tone around the room. For all identical speakers, the tone sounds exactly the same in every channel, and with timbre matched, it should be very close, but likely a slight difference. For mismatched surrounds, the difference will be much more. Matching the rears with the fronts will give you a better balance to the sound throughout the room.