The normals are important, because they are important for the efficiency of the render process.
Usually only the front of a face is visible (think of a cube). That means around 50% of all faces will fall out of the render process because they are facing away from the camera.
You need to keep this in mind when you model.
- You always model the surface of an object.
- You need to close the object, except on places where you are sure it is never seen by the camera.
But if you really need you can also switch on two-side. With the prize to loose the optimization. Even when you do this the normals should point into the same direction. Otherwise the shading might go wrong. Because shading is often normal based. This happens alot when smoothing is enabled.