Miles
>Tapping out could presumably leave some additional tensile stress on the lower surface of the chisel, where the steel has been distorted by the blows to bring it down to the target level. It is probably only a small distortion, but if the steel is hard, the stress associated may be large. This is the opposite of what happens in prestressed beams etc where a stress is engineered in to oppose the normal load in use.

I would wonder about the risk of promoting edge fracture when tapping out, as the resulting stresses in the lower surface should be tending to promote cracking at any scratch or grain boundary.

There's a reason why annealing is important after forging (or hardening) - I'm not a metallurgist, though, and it would be interesting to see if any here can comment.

I'd go with grinding, myself, to get that hollow moved back. The comments above about tapping helping to sell more chisels does rather suggest that it is a risky approach, and I cannot see any significant advantage.

Miles