Hi gang,
Some hammers will be ruined by putting tacks into them. Some hammers might be improved slightly . . .
As for value, it's simply a matter of whether the hammers have been destroyed by the rather large hold in the tip.
When I was a teenager I did this on our family's old upright - hey, my first voicing job! It was fun to goof around playing with the tacks on, then when I took them off the piano was suprisingly mellow sounding, and not too even. It was such an awful piano, I didn't care much at the time.
Doing this to a fine piano would be very damaging. Unless you want to leave the tacks there, in which case it would just boil down to a matter of taste!