There's nothing worse than getting out the front door just to realize you left the keys inside. If you don't lock your door before you go out -- and don't have a self-locking door -- then you'll never lock yourself out if you always lock the door from the outside. It depends if you're more the type to lock yourself out or forget to lock the door, which one do you plan for? I'm a lock myself out type, so I protect against that and always lock my door from the outside.