I think Brian's argument is one of purism. However, I don't think there can be much departure in sonic character of the rears from the mains as there is little to no voice in the rears. I also think you would need great difference in sonic character of the amps being used to tell any difference from the front to back. Across the fronts (L/R/C) I definately see the rationale but not towards the back, but this is just my take and I could be wrong.
If it were my money, I would focus on the front three and leave the rears for a much later endeavor.