If using the toilet I tend to lock the door - I'm not quite as obsessive about this since I had kids, but I do still lock the door often. if I'm just showering then I don't bother - mostly because there is usually a child in there with me!
No. We brought our kids up to not be ashamed of their bodies. We don't even shut the door. If people need to come in to talk or wash their hands they can. Besides, if there is a fire, you need to get out quick. Locked doors can be a real hinderance if there is a fire or other emergency where you need to get out quickly. If someone passed out or had a heart attack, no one, including the ambulance service could get in. Its not safe.
Never. I smashed the locks off the bathroom and toilet doors. I hate them. Never lock yourself in a small room, folks. A friend of ours had a heart attack in his shower and fell against the locked door . . . too late by the time they got it open. I will never forget the terror I suffered as a small child when I pushed the button on the toilet door to see what it was and got locked in. It was a rarely used old button lock and I could not get it open for ages. Major panic. I got sick of telling his grandchild to STOP locking the doors, so I smashed them off. Easy.