I've had a lock on my door since I've had my room, only time I used it was when my brother would chase me down the hall, and I wanted to keep him out.
![Smilie](/tfp/images/smilies/smile.gif)
The general rule of Knock and wait was the standard in our house.
However, locks can be a good thing, if on the OUTSIDE of the door. One 3 yr old kid I watched managed to lock the door on the inside, and his dad couldn't get in without breaking the door. But move the lock to the outside, and viola, you've got a way to keep the kids in the room during naptime!
Me personally, I would hope to teach my kids to have enough respect for each other, so they don't feel the need to lock their doors for privacy or security reasons.