@bunsenburner:
Yeah, unless someone had crappy filters on the furnace, it is unlikely that the outgoing ducts would have much problem,it is the returns that have the problem. There can be legitimate reasons to clean out ducts, if they had problems with mold or something like that, or with pets, but a lot of the services out there are scams, they basically vacuum out the ducts at the opening, and then if they have access to the ducts coming into the furnace they vacuum there, but they don’t really clean the system, and charge a lot. There are filters you can put on the return ducts, which are a bit of a pain to replace, given there are likely a lot of returns in many houses, but that would prevent stuff from getting down into those ducts.
A forced air furnace generates nothing near the air flow of a vacuum cleaner. I recommend the curious take the grill off a could of your vents and look as far as you can with a strong flashlight. If you’re comfortable with what you see, let it be. If you see a lot of crud/detritus/whatever, you have options. This is probably not a one size fits all issue.
FYI, negative air flow is used in BSL3 labs to keep aerosols of pathogens such as TB from exiting the labs. It is not as powerful as a vacuum, but it gets the job done. My point is that you do not need the power of a hurricane wind to keep light particles from settling into the ducts. Unless you physically drop larger stuff into the vents, dust will generally not accumulate in them. For me, I take the grills of my vents every time I do deep cleaning, and I can not see any crud other than baked on paint etc. that was left in there since the time our house was newly built, even though I have cats… 
But their hair is really light.
It is definitely not one size fits all.
When we bought our current house, I got a bid on duct cleaning. I figured we might as well, since we had had the floors sanded and re-stained, and I didn’t know if the former owner had pets. Our house is big in terms of square feet, but I still was shocked at the bid. It was several thousand dollars! I passed.