Brian, I'm not a HVAC pro either, but with regards to closing the supplies or closing a return, my understanding is this. If you close a supply to a room, it forces the air to exit some other place that has less resistance. The point is though, that air goes someplace into the house. With the return, if you close that off, it eliminates air from entering the system, so less will be pumped out of the supply.
So by closing a supply, 100% still exits the system, but by closing off a return that 100% may now be reduced to 50%.