We have house ruled that you have to have LOS on any target or starting point of any area effect. So for example you might not be able to see a model, but if you have a ranged spell that targets an area you have to be able to see the center point then lay the template using the point you can see as the center/starting point.
It's like throwing a grenade, you might not be able to see the guy in the building, but if you throw it in the window near him he can get caught-up in the blast.