I never open my locked door to a stranger but if it's a big guy out there (I don't want my door kicked in), I open my window by the door and tell them "I don't open my door to strangers" Most understand and say so. The rest get to watch the jalousie window close. My DH just blithely opens the door.....but I have finally convinced him to check before he does that. He's such a trusting soul.
I think it was about 25 yrs ago that I had an epiphany.....just because someone wants me to answer a doorbell or knock or the ring of the phone does not mean I have to react to this invasion on my privacy.