I have never come across a bleed valve that goes all the way in until
the head is against the radiator. They normally have a pointy bit that
seals on the end and making contact with the head would stop them
working. Therefore a small chamfer makes bugger all difference to
contact area and does make them easier to make and use.



Removing the chamfer increases the chance of all the square being in
contact and not twisting the end off or ruining the key.