I always place my thumb in front of the hammer when dry-firing a gun that isn't mine as well.
Per Ruger's FAQs on their website:
Q: Can I dry fire my Ruger revolver?
A: Yes. All Ruger revolvers can be dry fired without damage, and dry firing can be useful to familiarize the owner with the firearm. However, be sure any firearm is completely unloaded before dry firing!
I dry-fire my SP101 sometimes and I've never pinged the cylinder. I would not do this with my S&W rimfires though.
From S&W's FAQs:
Q: Can I dry fire my Smith & Wesson?
A: Yes, except for the .22 caliber pistols which includes models 22A, 22S, 422, 2206, 2214, 2213 and 41.
.22 caliber revolvers such as models 17, 43, 63, 317 and 617 also should not be dry fired.