If I thought there was an "Entertainment Rate," I'd ask for it when booking a room even if I didn't have a single valid card. When I used to do this, only one hotel in the whole world (the Sheraton in Stockholm) ever demanded the card and stuck with the demand. Fortunately, that was one of the times I had one.

A few others asked at check-in, but I just said, "Oh, I have to get it out of my bag, I'll show it to you later," and that sufficed.