During wintertime, the furnace air has been drying out the family
sinuses. Also, we have been sick a few times. While investigating, I
thought maybe it was a furnace issue.

I had a reputable furnace man out. He suggested having the furnace
cleaned/AC cleaned and each air duct cleaned. For a total of $500.

I have heard that the furnace/AC cleaning is a good idea. I have also
heard that duct cleaning is just a scam, because the filter picks up
the dirt that would be in the duct.

Any thoughts on that?