Which receiver are you using? If the receiver does not allow for a 6.1 or 7.1 set up, I wouldn't run a 6th channel at all. If the receiver does do 7.1 usually the manual will tell you to use the left or right rear channel as the 6th speaker, the receiver will do the rest.

I run a 6.1 system, 3 QS8s, and found the rear channel to fill in the gap nicely between my side surrounds and really envelop me in sound, as I have a large gap between my side surrounds as well.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70