Well you gotta understand the discharge curve of the batteries, lipo family of batteries hold a higher voltage at discharge better than NiCads or Nickle metal.
now if you regulate to 6. Volts on the receiver, and 5 volts on the ignition but recharge at 7.1 volts on the battery you've got a cushion there.
look at a smart fly board when get to more than 1 servo on a control surface