Or are you saying servos should always be powered by 6vdc because that way they deliver their highest speed and torque?
I said "If the servo is expected to hold/move any real load, 6v will be much better. Check the torque specs for servos operated at 4.8v and 6v". If a person has a choise of voltages to drive a servo, I'd suggest selecting the voltage near the high end of the voltage range. The below servo has the specs for operation at 4.8v and 6v. I see a noticable difference in performance with my servos just between 5v and 5.7v.