Duct cleaning: Advise getting a good job?

Not necessary. The dust collected there over the years, and, unless you so something to greatly increase the airflow through tthe ducts, it's going to stay right where it is.. If the dust were going to blow into the house, then that's where it would be, not settled in the ductwork.
Consider it a second stage filter. Water treatment plants use the same method, it's called Settling. -- Email reply: please remove one letter from each side of "@" Spammers are Scammers. Exterminate them.
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