I use shadows to ..push words, graphics, letters, what ever ..toward main traffic and add dimension to a graphic.
When adding shadows I consider main traffic and viewing from.
I use light source for shadows for art like doing a painting when I can choose my light source.
Shadows are meant to push the graphic closer to viewer and give dimension in advertising, also careful in use of the shadows
... example red letters on white background a light gray shadow is best or it is decorative using a lighter color value then red like yellow and many times can crowd the copy making it hard to read, this is why care in adding a shadow is needed.
Many times you will see red letters on white back with dark shadows like black or dark blues or orange or other colors of likeness and just destroy the copy and effect.