Just to share my maintenance with my speakers:
Weekly: Im using clean/dry 2" paint brush to clean speaker surface including the cloth grills
Monthly: Cleaning of back side of the speakers including speaker connectors.
Wipe speakers body with "Pledge" to retain natural finish of any wood veneer surface
We need to clean our speakers religiously especially the cone because of very humid condition in our country, heavy dust stock on the cones tend to create permanent stain especially for the paper cones. Regular cleaning of the speaker grills means no need to wash them with water & mild detergent which makes them dull color in the long run.