I am in the minority. No locks. Seriously...if a bathroom door is shut in someone's home would you walk in without knocking? I guess I am astute enough to know that if the door is shut it means someone is in there, and I only hang around people who are apparently as astute as me because I have never had a problem with guests either.
When we moved into our house, one bathroom did have a lock on the door and our then toddler locked himself in the bathrrom and couldn't figure out how to get out. It was a PITA to get the kid out! When we renovated our bathrooms the new doors got new knobs with NO locks.