On Rx's, there really doesn't need to be a dedicated slot for a battery. If fact, some receivers just use one of their "channel" slots for power. If you need that slot for an actual channel, then you can use a Y--one end to the battery, and one end to the servo. You could actually plug a battery into every slot on the RX if you wanted--just use Y's to hook the battery and servos to the channels together. You would still have 4.8 v (if all batteries were 4 cell), but you'd have a bunch of mah possible!

Back to the question--plugging a battery in backwards. If it is a dedicated battery slot (such as found on some rx), then it shouldn't hurt--because there should only be two prongs inside of the slot, and plugging the battery in backward wouldn't result in any current being pulled. However, if you plugged it into an operating channel, where there are three male prongs inside, then plugging it in backwards could have some negative consequences.

I believe most rx's have reverse polarity protection, but I could be wrong on this.