No matter what lighting you use, it's always a good idea to look at the shadows. Whether you want a more shadowed/dark photo or you're looking to capture a well-lit portrait, you want to ensure you don't have any of the shadows hiding your subject's features. Tracking the shadows will also help you place the lights (or position the subject, especially when using natural light).
You can use reflectors or secondary lights to help soften shadows, but you still need to be aware of the shadows and which you want in the photo (and to what degree) to make them useful. To little shadow will make a person's face look flat. Too much can mask features. It's not just about how much light you use, but how that light interacts with your subject. Some shadows can be flattering, while other shadows will give you the opposite effect.
If you have reflective materials in the shot, you need to be aware of how that will alter the lighting and shadow. If you are shooting at glass/clear surfaces, you can use shadows to change the reflectiveness of the surface (shadow on the surface will let you see through, light hitting the surface will make it more reflective. When the light is behind the clear surface, the opposite effect is achieved).
