Hi Reuben,
If you are doing light trails, you would be better off focusing on the entire scene (likely the background – as per the image above). If you are photographing a person in motion, you would likely make them in focus. But, this depends on your intent for the image too. With the boxing image above, the person is the focus of the image, so they are in focus. If you want the person to be blurred out more to show movement in an overall scene, then you would focus on the scene. I hope that makes sense?