If tube amp is left on, but not playing........

If a tube amp is left on , but not playing, is the tube still wearing out? If you think of the tube as a valve, and no electrons are moving through it, it would seem that there is little to no wear on the tube. This is a significant question for me, I am considering a SET for a system that will be used by my family. My wife has never shut off a light in her lifetime, and will not turn off the amp when not in use. (Those of you who respond with suggestions on how to change my wife's behavior have never been married!!) thank you......Mark