Duct cleaning...

"If no one in your household suffers from allergies or unexplained symptoms or illnesses and if, after a visual inspection of the inside of the ducts, you see no indication that your air ducts are contaminated with large deposits of dust or mold (no musty odor or visible mold growth), having your air ducts cleaned is probably unnecessary."

And also this:

"However, a light amount of household dust in your air ducts is normal. Duct cleaning is not considered to be a necessary part of yearly maintenance of your heating and cooling system."

Read the reference for yourself, and make your own decision on duct cleaning. It's your money. The article also provides useful information to help you evaluate whether the folks performing the work are actually taking the appropriate actions.

As for me, I stand by my opinion that most duct cleaning is not needed, and remain unpersuaded of its universal goodness by isolated anecdotal reports. (Homeopathic medicine has similar vigorous advocates). If you do not have air conditioning and do not have an add-on humidifier, there should be no moisture in the ductwork to support the growth of mold.

• EPA site on duct cleaning

Hospitals, schools, and most public buildings, nowadays, must have their duct work cleaned for state inspecions.

You are wise to have had your system cleaned in your residence.

GOOD FOR YOU!

Most do-it-yourselfers can really screw things up with their FIBERGLASS ductwork. The ducts will start eroding and have fiberglass floating around their house for years. That ain't good.
John,PA

Cover a spare pressure diffuser, floor or ceiling in your residence with a piece of clean panty hose. After 2 weeks of normal operations, see what collects.

I insist that this be done after the house is newely finished before I consider the house safe for HUMANS, and certify as such!

Most just wait until the ceiling needs repainted (dirty), especially in SUPERMARKETS
Their computers are the first indication for duct cleaning. The cooling fans pick up the debris and eventually the computers will burn-out.

The air velocity in a properly designed forced air system is insufficient to move anything but the smallest particles of dust, skin, and hair. And most of those will be caught by your furnace filter before the air is recirculated to the supply ducts. The ducts are probably quite clean.

Most/all of the dirt that you see at the supply registers falls in there by gravity. Get your shop vac and sweep it out if it bothers you.

One possible exception: if this is a new (to you) house, and the previous occupants were heavy smokers, or had pets, AND you or your family is allergic to those things, AND you do not yourself smoke or have pets, then it might be worthwhile, one time only, to clean the ducts. While I still think the shop vac is good enough, a specialist would have the special tools to deal with flex duct vs. metal vs. ductboard.