I dry-fired my S&W revos thousands of times without snapcaps.
It is warned that snap-caps should be used on wheelguns with the hammer mounted firing pins. My old model 65 S&W service revolver is on the 3rd firing pin. The new 65 I picked up in '99 has a frame mounted pin and a flat faced hammer. S&W said it could be dry-fired without any problems. I dry-fired(without caps) it 2,000 times the first few days to break it in, and thousands more since. No problems.
Snap-caps are pretty inexpensive. They can't do any harm, only good. I just went digging through my reloading drawer and found a handfull. These say "Armsport .38 sp-Italy w/357M, 38SP & 38 SW" stamped on the case head.