If a Li-ion vented in your carry-on on a plane I think that would make for a seriously bad time for everyone on board.
I can't see a Lli-ion venting sitting in a flashlight thats turned off. But it is best to pack lights that you can unscrwew either the tailcap or head and disable them to ensure they cannot turrn on.
I fly all the time, all around the country, and carry on one or more lights with me always. Never been stopped or asked about them, not even asked to turn them on.
Good point by MikeSalt on carrying extra Lithium batteries: be very careful with them. Put them in a sock, secure them in an envelope. Do not let them loose inside the suitecase or toilett bag. Always keep them covered and protected.