Most receivers and servos will work okay on 5 cell NiMh or NiCad but there are exceptions, especially on some of the older servos. Do not try 5 dry cells however as that will burn out some of the receivers and servos. Most receivers have an internal regulator that hold the voltage to the RF section down to around 3.7 to 4 volts and any voltage higher than 4 cells put out puts a load in excess of the design load on the regulator which will accelerate the failure rate.