Most systems started out with dry alkaline batteries = 6V and even today many 4 & 6 channel sets still provide a dry battery TX and RX box. However, whilst the 6V gives faster response and more power, dry batteries loose power rapidly as voltage falls.
Nicads start fuly charged at about 5.2V hiold for awhile then drop and hold 4.8V much longer than the 6V dry cells would average out. 5 cell nicxads often cause problems by being "overvoltage" and if connected straight off a fast field charger cause jitters etc and may damage a servo.
Rather than more volts - use more amps. e.g. 1100 Mah in ieu of 500Mah battery pack. e.g. a small car with a flat battery is easily started by truck (12V 80AH battery) parked alongside but the small car 12V 24AH battery) will not start truck. Rather than ballast in a model - use battery packs as per =
http://www.fly-giantrc.org/technical/parallelbatt.html
to balance a model.
see also the sections under batteries and chargers at Alan's Hobby Web Links
http://homepages.ihug.co.nz/~atong/
regards
Alan T.