Makers of modern .32 S&W and .38 S&W ammo are well aware that it can and will be fired in black powder era guns and keep pressures to the same level as BP ammunition. I have fired many smokeless powder loads in those old guns with no problems. Still, I would recommend that the old-timers not be fed a steady diet of modern ammo; a few shots should be OK, but 500 rounds on a weekend - no.
Edited to add: There are some old guns whose cylinders are so thin, worn, or rusted that they should not be fired, period. With anything. So use a little judgement. A well made S&W in good condition will be OK; a no-name with cylinder walls the thickness of tissue paper - use it as a paperweight.