There are several reasons not to lead with your helmet -- not the least of which is your own safety. Everyone loves big hits, but leading with your helmet is just stupid, careless, and dangerous to both you and the guy you are hitting.
Football can be football without these types of hits.
Oh by the way, it does not make any difference whether it is helmet to helmet, whether the opposing player lowered his head, or just helmet on the opposing player:
The helmet rule (2006):