In a 5.1 setup your surround channels are Surround Right and Surround Left. Don't assign any rears. Change the option to none for the rear amp assign. Your AVR will downmix the incoming 7.1 content to 5.1 if available. The content from the rear channels will be directed back into the speakers you have in your 5.1 surround config.
Hi Mustang, Serenity has this absolutely correct. I also have a Pioneer receiver (running 7.1 configuration). On page 44 of your receivers user guide in the manual speaker setup section, make sure you have the Surround Back speakers set to 'No'. For a 5.1 configuration, your QS8's get connected to the 'Surround' connections, not the 'Surround Back' connections.
The QS8's are quite forgiving on position, so even if they are closer to the back wall as regular surrounds I'm guessing they'll still sound good.
Hope that helps.
