While I cannot provide information on whether less than 300psi is actually harmful or not, I would like to add my two-cents about Glenn and his shop. It has been my experience that while Glenn may be opinionated and he certainly can argue like a lawyer he is also a kind person. My experiences with him as a customer have always gone well. He has given me fantastic deals on equipment, rentals and air fills. In fact just yesterday he fixed two pieces of gear and gave me a battery for my computer. Didn't charge me a thing. And No lecture.
Glenn is he kind of person that will treat you well as long as you are treating him (or his gear) well. If not, then he is going to stand up for himself, his shop or his gear. If I had rented Glenn's tanks and then brought them back in close to or less than 300psi, I would offer an apology or at least a heads up. Then, I think Glenn might recognize that his gear was being treated with respect. I also know that Glenn is the kind of guy who can apologize if he is wrong and who can look at his part in a situation...if he is wrong, he'll own it. If not, he is not the kind of guy to sit around hoping people will like him. He is honest and helpful and if you give him a chance and treat him with respect he would give you the shirt off his back.
That is my two-cents about Glenn. I was pretty sad to read that some people won't go into his shop based on other people's word of mouth. I recommend getting to know these guys before making assumptions.