I agree with the head neutral crowd.
Head down-the barbell could roll on the cervical spine (as LaRue mentioned)
Head up-places undue stress on the cervical region (who wants unnecessary stress?)
Sometimes I place an object I front of them to focus on so that they can keep the neck neutral.
Hope that helps.
When you try different types of squats it is advisable to keep their heads up to keep their shoulders straight and prevent them from slouching