Personally I eat as much junk food as I want, with one exception: don't eat junk food when you're hungry. One piece of chocolate isn't bad but if you are hungry

Personally I eat as much junk food as I want, with one exception: don't eat junk food when you're hungry. One piece of chocolate isn't bad but if you are hungry it's more likely that you'll want to eat a whole bar. Just make sure that you always have an alternative for when you're hungry (fruits for example).