I rarely answer the door period, because 99% of the time it's some asshole trying to sell me drapes. If I'm feeling grumpy I'll yell fuck off.
the last man on earth, sat alone in his room. there was a knock on the door. (sorry its a quote, and sorrier still i don't remember who to attribute it to. probably harlan ellison, but that's totally a guess.)
I do not answer after dark except by prior arrangement. I have a sign on the front door saying so. In any case I have security screen door - quite common in Australia.
I think the worst thing that's ever happened to me opening the door was someone tried to talk me into looking at the Book of Mormon. And she was kinda smoking, so I briefly considered it.
my worst experience was probably when i assumed it was a mormon and it was actually my mom. and my apartment reeked of weed. some good came out of it though, it was years before she stopped by unannounced again.