The Shadows Know
When adding shadows to your drawings, always go with what's graphically right. Forget tepid, anemic notions of "realism". Save that for medical illustration. It rates a far second to composition in comics and illustration. If you want to create rigid realism, take up photography. In addition to being beautiful in and of themselves, expressive shadows and lighting will serve your composition, and can also generate mood and atmosphere. For example, dramatic or distorted shadows can imply a sense of impending doom, violence, or entrapment, etc. And, when used expressionistically, such visual techniques can suggest or reinforce a character's psychological state. Just ask the Germans. Visuals can tell a nonliteral, poetic story all by themselves. The shadow is the artist's friend. If it feels like there should be a shadow there, there probably should be a shadow there. Or, as Wallace Wood once said, "When it doubt, black it out."