marvi,
I have seen some digital servos that can not handle over 6V. They were damaged when connected to a freshly chargerd 5 cell NiMH pack. Interestingly the same servo ran great with 4 alkaline cells. After speaking with Hitec tech's it turns out that this can happen with some digital servo/reciever combos. If the reciever does not have a regulator then the voltage to the servo can excede 6.0V and possibly damage the servo. All three examples were in Futaba RX's with one Futaba sevo and two Hitec servos.
Your best bet will be to stick with a servo that gives you specs at 6V as this is an indication that it can handle that voltage.