Dirty blades can make ceiling fans shake and wobble. To keep them running smoothly, clean them often.
An out-of-balance or under-lubricated ceiling fan increases the wear on the fans motor, which shortens its service life. An out of balance ceiling fan can also lead to safety hazards, such as loosening electrical wiring, or even the entire fan falling down.
Ceiling fans should be cleaned, inspected (and possibly lubricated) once per year.