IIRC if the player is acting in the capacity of a runner, you can contact another players helmet with your own. It's the defenseless players ( rcv'rs at the moment of the catch, QBs sliding in a baseball style) that you can't lead with the helmet. Least ways this is how it has been all season. IMO they wear helmets for a reason, the officials need to figure out which reason that is. Is it like long hair hanging out from under the helmet (considered part of the uniform) and can be used or not.
jusy my $.02