Unless you're totally sure that you don't want/can't fit the center rear channels, I will not get a receiver/processor that doesn't do 7.1 processing. If you've got the room, for movies encoded with a 6th or 7th channel, having that 6th or 7th channel is definitely a more pleasurable experience.
With that said, the rear centers have very little information and so I definitely won't worry too much about it. I would use the funds to build the best 5.1 system you can. You can pretty much throw any old speaker in the back and it'll still improve your experience IMO. Timbre matching is really not needed for those rear centers, although obviously, having speakers that match the rest of your system will make a subtle difference. So you can always add those rear centers as budget permits.
With that said, the rear centers have very little information and so I definitely won't worry too much about it. I would use the funds to build the best 5.1 system you can. You can pretty much throw any old speaker in the back and it'll still improve your experience IMO. Timbre matching is really not needed for those rear centers, although obviously, having speakers that match the rest of your system will make a subtle difference. So you can always add those rear centers as budget permits.