1 or 2 Rear surround speakers for 6.1 system?

I'll soon upgrade my HT system to 6.1 from 5.1. I'll have the usual L+C+R speakers in the front and 2 side surrounds and plan to add another one for the rear surround. Now, is it really necessary to have 2 speakers in the back or can I get most of the benefits with one channel/speaker?
I put 2 in the rear and two on the sides. To me with recreating the theater experience being one of the primary reasons for this system, you get more robust sound with two rears. I prewired for the future 10+ system with only one rear center because the spec. implicates this. Otherwise I believe for the present 2 rears work quite well.
I would use one for 6.1 and two for 7.1
As far as I know there is no real 7.1 surround system. I've read it is only a manipulation of processor manufacturers,taking some info from the side surrounds and directing them to the rear.
Muratc, 7.1 is a futuristic concept. It will allow for seven discrete channels of playback. Pre amps are being sold now with this capability but like everything else in this hobby waiting on the technology could take awhile. For now 7.1 will indeed rob info from the rears to feed the side speakers. In DD6.1 EX or DTS EX the sixth channel is in fact a discrete rear center channel. I doubt this will catch on but only time will tell. Personally I'm still perfectly happy with 5.1 surround sound.
I have a 7.1 set up now but I started with 6.1 and if money is not the issue for the extra rear speaker than go for it.I have done alot of different things in my home theater and the 7.1 set up works the best for me so far.Also be sure to place the two rear channel speakers about 2' or 3' apart from one another.