I am sure I have seen 4.8V only servo's? hmm cant remember...

Anyway for 7.2V servo's they wire the power in from battery and not RX unit (by-passing regulator), only the signal comes from RX unit.

My Ko Propo servo did come with 6V and 7.2V instructions but I didnt take much notice of the 7.2V stuff, but it was a lot more then just plugging it into a receiver.

6V Vs 7.2V KO Servo:
PDS-2123 FET 7.2 V 131.9 oz. 0.06 S @ 60°
PDS-2143 FET 6 V 111.0 oz. 0.08 S @ 60°