Several people have mentioned changing the angle of the hammer nose to match that on the nipple, as many hammers don't, the difference is visible in the indentation on the hammer nose. Remove material to the bottom of the dent.
The case hardening on these parts is so shallow, since it's for color only, that removing metal isn't going to make a difference. If the case hardening were deeper the nipple wouldn't indent the hammer nose to begin with. The nipple is the part you don't want blossomed too oversized for the caps to fit over. Niples are cheaper than hammers, true, but that's not a benifit if the caps don't fit.
Were you to reharden the hammer, you would probably just make it brittle, and then it would damage nipples as well as being more breakage prone.
The hammer nose should just barely touch the nipple face so when a cap is on it there is enough thickness to fire the cap, and that is how the Ruger is set up. The hammer nose should also be parallel with the nipple face.