I have read hundreds of times not to dry fire a rimfire S&W because it could hit the chamber edge and ding it.

This doesn't make sense to me. There is more than enough clearence between the firing pin and the chamber edge, I have measured it.

Can anyone tell me why this statement continually comes up?

Now I can understand on a non recessed barrel and a unlimited firing pin travel. I have seen several 22 rifles this way. But a S&W has limited firing pin travel.