Should I tip the air waitresses?

When flying I was wondering if you should tip the air waitresses. I tip regular waitresses, but since these guys can fly the plane in an emergency should I be tipping them also as I get off the aircraft for serving me small bottles of liquor and treats? The last time I was at Chili's the waitress I had was very good and attentive, but I am pretty sure that she couldn't fly a plane in case the regular pilot has a stroke during a flight.