I would never answer the door not knowing who it was. If its family, friends, neighbors i trust then yes id open the door. If it was a stranger, no way, unless they looked in distress in which case I'd call the police and tell the person hang tight the police are on their way. I'd have protection on me though just in case.
That's what i WOULD do, but...that being said I did have someone ring our bell at 3 am, and I let him in without question.
It was a 13yo from several streets away. I did not know him but had seen him before outside, but didn't know him or the parents either- but he was at our door barefoot on a cold night and crying. I didn't think I was in any danger.
Apparently without knowing any real details there was some sort of abuse going on, he finally had had enough and ran out of the house, ran for a few streets and then didn't know what to do so just rang the doorbell of the house he had stopped in front of. (We just kept him warm and called the police and then they took it from there.)
After that I thought people should definitely be careful but don't discount the idea of someone truly being in need. Rely on your judgement but have protection in case it's needed.