That rating on Spektrum RX's of 3-9v is bull-poo, I've known people blow them up trying to plug a lipo into it.

When I helped make a 3-Servo lipo ant, I had a voltage regulator that stepped down 7.4v to 6v and powered the receiver, the positive wires from all 3 servos went in before the regulator chip (FET). Spektrum RX's may struggle with that unless you have a large battery and a high rated FET, the one I used was 1.5A.