[Al Skierkiewicz]

All,
I like the approach given in the post above from SuperDanman when you know that the battery voltage is high. Although the parts count is high and it takes space to build, it is a very reliable way to insure the correct voltage is supplied to the servo. However, when the battery voltage drops below about 7 volts the regulator starts to drop out of regulation and it essentially turns into a resistor.
A simple diode (1N4001) in series with the battery lead will drop the voltage by 0.6 volts, cathode (banded end) toward the servo. Add two diodes in series for a 1.2 volt drop. The diodes are easy to install in the PWM cable with a little solder and a piece of heatshrink.
However, FIRST has been using servos for years connected directly to the output ports of the controller since those PWM outputs are at internal 5 volt reference instead of battery voltage. We have used servos in the past to initiate the action of small doors for instance. (Our Magic Tongue release of last year for one.) We have also used servo to initiate the change from brake to coast on speed controllers, where the servo actuator contacts a micro-switch which in turn is wired to the brake/coast jumper on the speed controller.