I've never had my ducts cleaned, and probably never will. I figure, if it ain't broke, don't fix it. And I've never had problems.

I vacuum the ducts out with the hose every so often, as far as I can reach. I change the furnace filter every month or so in the winter. My husband oils and vacuums the furnace once a year.

Never had any issues, and my furnace is probably 40 years old.