According to the EPA Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space.
However, the National Air Duct Cleaners Association (NADCA) recommends air duct cleaning every 3 to 5 years.
If there are rodents in the air ducts or mold or some other air quality problem related to the air ducts they may need to be cleaned.
Air duct cleaning is sometimes sold along with furnace tune-ups. Furnace inspections and tune-ups should be part of yearly home maintenance. In most cases, duct cleaning is an optional add-on.