All of the above, I think . If its a receiver battery, that is one whose voltage does not exceed your receiver's, than yes you can. As you may know the idea behind the BEC (Battery Eliminator Circuit) was to remove the weight of the receiver battery. If you have a large aircraft that doesn't care, the incentive is all your main battery energy goes to powerplant, but usually the servos and receiver draw so little it doesn't seem to be worth it. Some would still run a receiver battery through a regulator...