Josey,

Think about it for a second. It's kinda like a dam in a river. Without the dam the water will always flow down hill, but having a dam is like having a regulator in preventing the free flow to the lower side of the river. Add to that the comparison to when you use a fresh battery to jump a low battery to start a car.

Since you can plug a battery into any unused port of a receiver bus and power the servos from either end, it becomes clear that a receiver has no directional voltage restrictions at the bus ports.