[Bunyip]

I was taught when starting out, and have been consistently informed that the best practice is to leave 25-50 PSI in the tank, rather than running it dry. As for causing damage, my understanding is that tanks are made from steel, oxygen is a very reactive chemical, if you run it dry there can be air exchange allowing humidity etc. to intrude. Particularly if you leave the valve open...

If this is incorrect I'd certainly be interested in learning what the real deal is.