Well this is certainly a hotter topic than I expected!
For better or worse, I got the tap and die out yesterday (after about half of the above responses but before seeing the other half) and went through the heads - it didn't really feel like it was cutting but no way to be sure. Used the die on the bolts and they certainly screw in cleanly.
It would be really interesting to do a "snugness test" or something: use a normal cutting tap and die on say, half the holes/bolts and a thread chaser on the other half... and then see if they feel and fit any different.