I use 5 cells NIMH and NEVER use a reg ( wouldn't have one as a gift )
some guys are running 2 cell lithiums -with NO regulators - on rx or servos
I won't mention brands or names - but they work just fine -and the servos really work !- This is not an endorsement to try it on your own stuff .
The 180 JR retract servos sometimes would not work (they are POD type) unless the ATV was set correctly -On 6 v, mine always worked fine .