The question in the subject line is inspired by a Ms. Manners column where someone wrote in concerned about guests that come to their home and instantly want to use their computer when they see it.
In my case, since I live at home, its not much of an issue. Mainly because, if someone is staying over, my room is the guest room and as such, my computer is off. Also, until last week, the computer was in a lockable armoire (never locked, wasn't an issue with a bed right in front of it). Also, most of the guests that we have over are family members, generally my grandparents on either said, occaisionally an aunt/uncle. Most are not as computer literate as us or as tied to a computer, and so don't need access.