Like Robert already replied, it's just a choice, if you look at most name brand servo, they post the torque specs for 4.8V and 6.0V and of course at 6V the servos provide more torque. Same goes for name brand RX, they support either voltage, but check with your RX brand to make sure it supports the 6V before you try it.
. here you can see the Futaba 4.8 and 6V torque for some of their servos. Again this is for Receivers and the transmitter has nothing to do with this part.
Originally Posted by RGS2010
Do all Futaba TX's & RX's run on 6V ???