I've always fed in their enclosures, and I have no problems.
Snakes only associate your hand with food if that is the only time you open the enclosure, (or if you've been handling rodents).
I don't handle my snakes as often as some do, but handle mine when I clean cages, weigh them, check them, etc., but most of mine are not aggressive. I say that, because I have a couple, like Zebra Jungles that are just plain evil, no matter what. LOL
But having fed inside their enclosures, regardless of age, etc, they do not bite me.