I like to have an on/off switch. All mine does is cut the power from the ESC to the receiver. I feel safer killing the power to the receiver (and throttle) when bringing the plane to and from the flight line. I will add that it is a bit of a hassle getting to the battery connectors on my plane.

The MAJORITY of on/off switches DO NOT KILL THE POWER TO THE ESC. The ESC will drain the battery even with the switch in the off position.

If you want to kill the power to the ESC via a switch, you will need a switch capable of handling AT LEAST your MAX AMP DRAW PLUS 10% and you will need to wire it into the positive lead (usually red) between the battery and the ESC.