I've only been instructed once to turn them off. However, they are electronic devices unlike the self-powered versions that are used in international F or C (huh? they are the same thing, except the airline versions get power from the plane, vs the standalones, which are powered by 2 AAA batteries). I agree that they probably won't interfere with anything, but neither will a game boy or small electronic bowling game or similar things, and you have to turn them off also.

-David