This is not a very common myth. There are ill effects of feeding your snake in it's enclosure. The only time it causes the snake to be aggressive is anytime you open the enclosure. Say you get the snake out twice a week, you will most likeyly be bit at least once. So you should feed you feed it once a week. Your snake is going to mistake you for food and you could end up hurting yourself or the snake. Please think people!
I feed 40 ball pythons weekly in their enclosure. They never attempt to bite me when I go into their enclosures the other one or two times a week. Do you know this from your personal experience? My experience says that "theory" is false.