circuit design options between switching Servo GND vs switching vcc
Strange thing: why do you need to switch the servo power? the control pin usually is enough (some servo simply shutdown when there are no pulses; it's not guaranteed). Also you'll lose position keeping which somewhat negates the advantage of using a servo.
Anyway. Kartman is right, you'll risk to try to power up the servo thru the control line, unless you are really careful and put it in 3-state. I agree that a better solution would be to use a P-channel or some kind of load switch (if your servo is logic powered, as it seems, a P-channel is extremely easy to use).
As for the flyback diode, most probably it's already integrated as a part of the servo bridge. It doesn't hurt to be safe however.
I suspect the control pin could send back some bad transient on power on/off of the servo (just paranoid); check with a scope and in case protect the GPIO with a zener or something similar.