Re-keying after tenant change?

[Uwe K.]

Wondering if anyone is re-keying or changing locks when a new tenant moves in.
Are you never doing it, or always doing it, or only if not all keys are being returned. Which then begs the question: How do you know all keys are being returned since there might have been copies made.
What of common area doors, like the back exterior door leading to a hallway of a duplex.
If a new tenant requests a change, do you charge them?