If it has not been done in a since 75 then it isn't a bad idea. If you have allergies or respiratory issues then definitely. Other then that I have never been a huge fan of duct cleaning, the BS 2" layer of dirt you see in adds is bogus. Most homes don't get accumulation like that unless the home is filthy and the filter catches most of the stuff before it gets to the supply side of the system anyway. Have it done periodically, don't let it be something that you lose sleep over.
I agree with Emperor, it might be wise to get them cleaned. But usually if they are dirty, there is underlying issue and cleaning them is just like putting a bandage on. It doesn't solve the problem.
I'd like to get a reasonable estimate to do mine even though we're the second owner and the house was never smoked in etc...I'd like to just cut out as much general dust build up as possible. Good idea to consider doing so..certainly can't hurt the air quality! JL
I would suggest you have the duct work cleaned every 2 years at least.
More if you have allergies. Once the ducts are dusty it will blow though the complete house.