Using 7 chanel processor with a 5 chanel amp.

I was just forced to upgrade my 5.1 processor to a 7.1 because the old one would not pass HD digital signals.
I still want to keep my 5 chanel amp, as I am in no way interested in 7.1
When hooking up the pre to the amp, should I just leave the two back chanels hanging and just hook up the other 5.
The manual for the new processor didn't say anything about that, it just assumed you are using a 7 chanel amp.