A member PM'd me and asked if it was a problem that he didn't remember to log out. Here's the reply, I figured others might find it useful, too.
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There is no harm in that, and actually if you are on your own PC, you don't even need to log out. Only when you are on a shared computer, should you always remember to log out. When you log in on a public computer, you could choose "remember me? - no" when you log in to have yourself automatically logged out when the browser is closed. If by chance you forget this, it is a good idea to change your password.