You may consider having your air ducts cleaned simply because it seems logical that air ducts will get dirty over time and should occasionally be cleaned. While the debate about the value of periodic duct cleaning continues, no evidence suggest that such cleaning would be detrimental.
Provided it is done properly.
On the clean side...
Duct cleaning is recommended every 3-5 years depending on your living conditions. You should consider having your ducts cleaned sooner if you've answered "yes" to any of the questions below: