Will you open the door to strangers?

If somebody knocks your door, but you have no idea who he/she is, will you open it and ask? Or you would just leave it alone? I have moved in where I live for half year, I always alert what is outside and I definitely won't open the door if I don't know the people especially when Idon't expect anybody is coming, it could scare the crab out of me! When I called the maintenance people for fixing computer or phone whatever, then I prepare somebody is coming and I will make sure I see their truck and notice they are really the right people. Maybe I watch too much movies about people rob in your house or do bad stuff to break in your house... that makes me worry about this.