Fret Hammers
Fret Tools
I never hit a fret with a steel hammer.
All my frets get tapped in with this plastic hammer:
It's the lightest Stanley model with hard yellow faces. I find it's important to get the hardest plastic faced hammer to deliver the sharpest blow.
If I do need a really solid blow to seat a fret, I'll use my two ounce ball peen hammer, and protect the fret with a 1/8 inch thick steel plate:
The steel plate is not especially flat, but I did file the surface to make it reasonably smooth so I can hit the frets as hard as I want with no chance of denting them.