ORIGINAL: Rodney
All the better receivers have a voltage regulator built in which limits the voltage to the rf section to about 3.8 volts (never measured the exact value) and that is the voltage the active part of the receiver sees as long as the voltage on the input is that high or higher.
If you are running a 4.8volt or 6 volt pack your voltage cant drop that low (3.8v) so you would have lost servo control long before your receiver lost juice.