Dry firing will not hurt an S&W. The old brochure/instruction sheet that used to come with the revolvers actually suggested it. They did not mention needing snap caps. I have never used these devices, and I have never had a problem. Admittedly, I have only been dry firing for a little over 40 years.
Paragraph 7 in the "Helpful Hints" section of the revolver brochure says, among other things, "practice dry shooting with empty gun." A quick Google search for this quote led me to two such brochures, one for a Kit Gun (rimfire) and one for a 686 (centerfire).
The current S&W web site also says dry firing is ok, although it says not to do it with rimfires, whether pistol or revolver.