I need to do the same thing with my new M&P's. I would think that dry firing with snap caps would be perfectly fine as this is exactly what they are for. Safely breaking in that trigger and practicing trigger sqeeze and control through dry firing. Dry firing extensively without snap caps is a no no on certain firearms but I don't know about the M&P specifically. I plan to do it on mine with the snaps/dummy rounds in place. YMMV.