I also do not use grain/treat to train this action.
Because, I may be somewhere that is not available. It was just a habit I got into many years ago from riding in the mountains.
I will give an apple cookie at the end of a good day and after a cool bath.
I only do it the same time, so the horse will not ask continually for a treat.
I am sure you all have seen those that nip for a cookie. I may also walk through the barn at night and give everyone a treat, just because.