It is abusive to drop the slide on the empty chamber of any auto. It is designed to be cushioned by the extractor being full of rim. HAVING SAID THAT, I have no idea how many times you would have to do it before you actually break something. I suppose you could think of it as store etiquette, like if you go to a Chevy dealership, and they DO let you test drive a Corvette, you aren't going to burn rubber and scorch the clutch, just because some other guy thousands of miles down the road is going to get to fix it later.
They asked Todd Jarrett in an interview what the average Joe should do to be a better shooter. He said dry-fire until your finger falls off. Hundreds of thousands of times. They asked him if it would harm the gun, he said; "I've done it literally millions of times and I've never had a problem. If it really gives you the willies, use a snap cap, but on any modern 1911, you're not going to hurt anything." (The idea is to make the feel of squeezing the trigger and feeling it break become sub-conscious.)