Tim,
I think it depends on the clicky tail cap - There are good ones that are momentary on and then full on, that require a truly deliberate press to activate full-on. The ones on my Streamlight TL series come to mind an excellent examples of the type. I've never had an accidental "permanent on" with the light, only deliberate. The perm on feature is nice when you need a bright light for longer than just a couple of seconds.