I think the card that provides "guidelines" for employees is a great way to re-emphasize the point for employees.
It is a genuine struggle for any organization be it small or big is to make your customer facing people greet customers cheerfully. And I think it is never enough to drive home this point and if having a card helps, so be it.
What matters to me as a customer, is somebody cheerful attending to my request at that moment and not someone who just wants to get you out of the way and do something better.