Here is a quick, little video on how I shadow. In any case, shadowing should not be excessive or over the top. When applying digital paint, it’s important to understand how natural shadows are cast.
In the instance of the eyes, less is more. You should realistically be focusing on lash lines, under eyes, and inner corners. Seldom to I ever graduate past the orbital; I don’t feel it’s necessary, and it tends to add an unrealistic flair to the image.