When was the last time they were cleaned?
Are the furnace filters changed regularly or cleaned regularly? (several times per year)
We have cleaned the ducts in all our units once since we purchased the rentals in 2001.
On our personal residence, we had them cleaned twice, the second time because of a musty smell (over 5 years apart), and the duct cleaner said the ducts were really pretty clean, except for one spot.
I'd say do it if it's been a long time since they were last cleaned. Especially since the HVAC tech said they need it.