Whether it matters for rear surrounds depends on your usage.
If you use your speakers to listen to SACD type orchestral recordings and need perfect timbre matching, dynamics, and detail then i recommend five identical tower speakers that can dig down to 60 hz paired with a pair of subs that can dig down to 17 hz =P
But i'm willing to bet you just want to watch movies
As long as your front 3 are the same as each other, and your surrounds are the same as each other, you should be fine. The angle to which the sound hits your ear has enough effect on timbre that it doesn't sound unnatural to have a slight variation in timbre. I wouldn't go with polar opposite speakers or anything but anything close will be perfectly fine. Our ears are a lot more sensitive to changes in timbre in front of us, which is why having the LCR be the same is a good idea.