If you use newer KO, Spektrum receiver you can use 7.4V 2S lipo directly. (sorry not sure abut futaba and sanwa). And don't forget to check the servo too as most servo donot accept 7.4v (only newer highend servo's do). If your servo or receiver donot accept 7.4V you need to add a BEC to lower the voltage to 6V....
Agree, if use 7.4v Lipo, better to use BEC to lower to 6V.
I have used 6.6v Life Rx w/o BEC and it works perfectly.
(with KO receiver and Futaba servo)