You should use the Surround A jacks for a traditional 5.1 setup. You might use the Surround B for multichannel music with direct radiating speakers and have Surround A for movies with dipole/bipole speakers. It's actually a really nice feature that's often overlooked. If you're doing the auto setup with the optional mic, the Denon will recognize that you don't have the back surround speakers hooked up and will configure accordingly. The manual is next to useless...poorly worded and laid out and all around confusing as hell. If you don't have the optional mic, I'd recommend getting it as it will certainly help set up all your speakers.
Jason