Like all motors, what voltage a servo motor will take depends on the load it is given. In several small models I have used servo motors as the main drive, in past times when small ESCs were either non existent or too expensive, I used the electronics as an ESC. The limiting factor for voltage was the servo electronics, which had as its main component a chip with a max rating just over 7 volts. A fully charged 6 cell pack might well exceed the chip rating.
The other limit on the motor was the very short shaft, awkward to get a good drive from. An alternative was to use motors recovered from dead CD players. The head drive motor was smaller, the tray drive a larger diameter, but both had a nice length of shaft.
