It is important to remember that recoating is easier and much less expensive than resanding.  Recoating entails scuffing the top layer of polyurethane and then recoating with polyurethane.  Recoating will take out minor surface scratches and dirt that have penetrated into the surface of the finish.  If areas are neglected too long and the finish is worn through, then sanding is required.  Floors should be recoated every year to ten years depending on wear.  A general rule is to begin with ten years and subtract 2 years for every person and dog living in the home.