I'll say that it depends.
Modern receivers can do a pretty good job with auto-equalization routines to help smooth out differences between speakers. If the speakers are close enough in sound reproduction to each other, then you can pull it off really well. I was running a MartinLogan Motion 8 center channel as a rear center channel for a while, while the rest of my speakers are NHT. When I ran the calibration routine on my receiver, I could listen and hear how that speaker went from sounding different than the others to sounding the same during the test tones after the receiver had adjusted the equalizer settings.
Also - the fronts and centers are, IMO and that of others, the most critical. You can have sounds going from one side, through the center, to the other side, like a motorcycle for example. If it changes pitch as it travels across your speakers, you'll notice. Surround speakers are not as critical to match against your front speakers.
So - front and centers are the most important to match, but your receiver might be able to fix any issues for you if they are close enough. The surrounds aren't as critical.
