When you open the door for the mailman, pizza guy, or UPS man what do you do?
Just do the same thing with the police.
Usually let the dog bark a bit, so they know there is a dog here. I'll peek out the living room window to see who's out there.
I grab the dog by the collar. As I open the door I shove my dog's head out there a bit so they meet my dog first. Usually by then my dog is wagging it's tail, but 2 seconds ago they heard a vicious bark. It's surprising how many people are scared of dogs.
Then I'll either stand at the main door cracked (apparently not a good idea according to jrumann59) or step out.
I like being partially behind the door and with the dog near by verses stepping completely outside and shutting the door behind me.
But having a cop in full uniform and patrol car knock is different than a stranger at my door.