You never have to open the door for a police officer unless he has a warrant, which you can ascertain with the door closed. What if your neighbor called and said, "Hey, I believe they're selling out of the house next door. The police are going to come knock on your door. If you open the door and they smell, then they have probable cause to get a search warrant to search your home. You actually are smoking weed. Don't open the door for the police." In the case where you just happen to open the door cuz you don't know who's at the door, you don't have a peepphole. In that case, I would step out and shut the door behind me and ask what the police officer wanted.