Guys,
I think the basic twist is you need two ways to kill the engine for safetys sake. Throttle trim is one, and a choke servo acts as two. Alternativly you can put in a switch to kill the igntion. How you control this switch is up to you. Optical, or even a plain old radio switch with a servo to turn it on and off works.
I don't know what the rules are, but the general rule is to have two seperate ways to kill the motor.

ALSO, please be sure to set your fail safe so if you turn off the TX (or loose its signal) you go to reduce throttle. You have a 50-50 chance of this being right if you don't check it, if you reverse the servo the default may be to full throttle, so be sure to check it.

Lastly, the kill switches only kill the motor if you loose power to the RX. In all cases this is a good thing IMO.
RickP