You do indeed want to timbre match all your speakers and it does make a difference. If you're budgeting, then go most to least expensive front to back. If you need to cut corners due to budget then do it in the 6th and 7th channels. The side surrounds, 4 and 5 are indeed very important and you'll notice a quality difference.
how much did you spend on rear surround speakers?
I thought i would share my experience setting up my home theater speaker system. I had Dali Helicon MkII for my fronts and Dali Helicon center. my side surrounds were an old M&K from one of their early surround systems. I replaced the M&K side surrounds with Dali Helicon C200 and was very happy with the sound. I moved the M&Ks to a rear surround position for my 7.1 system. However, when i found a pair of Dali W200 on Audiogon i bought them. After setting up with Audessey from my Marantz AV8801 processor, i was extremely pleased with the improvement in sound. I was very hesitant to spend much on side and rear surrounds, but in my experience, the extra dollars to have matching voices from the Dali helicon line actuallly made a difference i could hear.