I believe that "Never dry fire a gun" is like an old wives tale. May have been true once, but not now. I have since heard like the only guns that are vulnerable to dry firing are some rimfires due to their design. Don't even know if that's correct, either. Still some gun dealers and venders were raised hearing this one, and WILL get upset if you do it. Instead of the disclaimer you offered, just ask them if it's OK if you dry fire it. Yes or no should be your reply.
Some people think I'm paranoid because I have guns. If I have guns, what do I have to be paranoid about?