Exception should be used for things you can't control within your program. Most of time, this means you are working with external files or an internet/database connection.
For user input, YOU should control it before you compute anything with it.
Of course there is a limit to what you can expect and you should stop before reaching the "armless person exception". We came up with this term when we were programming something with movement detector and one of our friend was over-protecting his code. So we told him: "Yeah so, did you control what happens if an armless person put his hand in front of the detector?". We got a good laugh with it :)
