It is fair to point out there is a very, very small percentage of material that will actually give you distinct sound to 7 channels and of those titles on blu ray I've listened to the difference has been so subtle I wouldn’t' go to great lengths to do 7.1 for myself and I have a very similar room size to you. In larger rooms as already mentioned it can serve a purpose to help fill the sound out in the room, but for the point of hearing 7 distinct channels I don't think the source material has a high enough penetration right now to bother.
But to say it another way. There is some 7.1 discrete content out there now, and there will only be more in the future. Along with PLIIx which can make 7.1 out of anything (not to mention PLIIz which can make 9.1 out of anything). Don't rule out support for 7.1 if you have even the smallest interest in using it in the future. At least run the wires, or make some allotment for it, even if you stick with 5.1 for now.
