Speaking from experience, for something to appear out of the fog of war in League, the center of its hitbox has to be in your champion's field of vision.
League's fog of war system is done in a way so that you either see something or you don't. You can't partially see something, like you can with some other fog of war games. You need to have vision on the center of the hitbox to be able to see and target it.
This works for everything, not only for monsters around corners, but also for champions in a bush, anything on the edge of the fog of war, and so on.
EDIT: I'm sure they could change it, but it'd end up being a mess. You'd be able to see champions who are on the edge of a bush, the game would have to partially render objects (like parts of a champion being visible), and so on. It'd end up being buggy, unclear, and affect gameplay significantly.