Air duct cleaning service providers may tell you that they need to apply a chemical biocide to the inside of your ducts to kill bacteria (germs) and fungi (mold), and prevent future biological growth. Some duct cleaning service providers may propose to introduce ozone to kill biological contaminants. Ozone is a highly reactive gas that is regulated in the outside air as a lung irritant. However, there remains considerable controversy over the necessity and wisdom of introducing chemical biocides or ozone into the duct work.

If you are replacing your air conditioning system, make sure that the unit is the proper size for your needs and that all ducts are sealed at the joints. A unit that is too big will cycle on and off frequently, resulting in poor moisture removal, particularly in areas with high humidity. Also make sure that your new system is designed to manage condensation effectively.
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. It is normal for the return registers to get dusty as dust-laden air is pulled through the grate. This does not indicate that your air ducts are contaminated with heavy deposits of dust or debris; the registers can be easily vacuumed or removed and cleaned.
Moisture can enter the duct system through leaks or if the system has been improperly installed or serviced. Research suggests that condensation (which occurs when a surface temperature is lower than the dew point temperature of the surrounding air) on or near cooling coils of air conditioning units is a major factor in moisture contamination of the system. The presence of condensation or high relative humidity is an important indicator of the potential for mold growth on any type of duct. Controlling moisture can often be difficult, but here are some steps you can take:

Yes everyone should get all their ductwork cleaned, because it does help, incame down with major allergies and had serval operations on my sinues because of this problem of air quality and dust, plus i almost died from one of the operations, i was suffering so much. Yes you can seal your duct system by buyind duct seal t instead of tape to tape all the corners of the vent system, tape wears out and gets brittle, the sealant you can buy at home depot, and you just brush it all over the connections of your ductwork, it comes in a container and it is like a thick paste and dries making a good seal for your ductwork.
I am a service technician [since '92].....and as to double filtering.....I have never recommended it. would be noticeably helpful if the fiberglass filters are the main ones used [which I also never long as system can deliver appropriate air flow with a pleated cotton filter...and any decent service tech can check the air flow for proper cfm]. Any particles small enough to pass through a pleated cotton filter are going to pass through a second filter as well....unless the second filters are so tight that the system is starving for air flow. Has anybody ever accidentally put two filters in their automatic drip coffee maker? What if cars used two fuel filters or air filters? I have told some homeowners to try using second filters at each register [the black thin filter material cut-to-fit....same type used in many window air conditioners].....but I only suggest this when I know they will still get at least 400 cfm per ton of air flow [350 absolute minimum] some homeowners know others who say it has helped and it gives them a peace of mind knowing they are doing something to help contribute to a solution, so it HAS to help [even though I think it falls in the "one-born-every-minute" category.....similar to duct cleaning].
If there is stuff in the ducts, the fact that it’s laying there and not being blown into the rooms is proof positive that it is hurting no-one. The 5-6 exceptions you cite in this article I’ll buy, but only with the caveat that at least some of those could have been avoided by proper protection (during remodeling for example) – and if asbestos/lead/whatever winds up in your ducts- the remodeling contractor should be the one paying to remove it.
I guess, I will start with saying that air duct cleaning is almost never needed. It is not a scam, but it is certainly something that uneducated homeowners get suckered into. In regards to air flow, a well balanced HVAC system should be able to deliver more than enough air flow. If you are not getting good air flow, it's because your system may be out of balance. Additionally, regular cleaning/changing of the air filters will solve about 95% of your problem. Though you would be surprised how many people either a. don't have a filter or b. don't know about it and leave it there for 10+years. a washable one should be cleaned every 6 months(spring and fall) and a disposable one should be replaced every 3-4 months.
Here in Frisco, TX…just had a tech come out to clean vents. I used a 30 bucks groupon for “complete vent cleaning”. He told me I have 3 AC units and groupon covers only 1. I told him to do the unit upstairs where the kids sleep. He decided on his own to do the downstairs unit. I yelled at him why he is not doing the unit I told him to do! He said they are all connected. I will clean two for you for the price of one… Oh really?

Since most states and municipalities don’t license air duct cleaners, you need to check their professional credentials instead, such as NADCA membership. EPA recommends all duct cleaners follow NADCA standards. Member companies must keep at least one technician on staff who has passed a NADCA test. “They have to pass rigorous testing to earn the certificate, and our code of ethics is very important,” Vinick says.
You may consider having your air ducts cleaned simply because it seems logical that air ducts will get dirty over time and should be occasionally cleaned. Provided that the cleaning is done properly, no evidence suggests that such cleaning would be detrimental. EPA does not recommend that the air ducts be cleaned routinely, but only as needed. EPA does, however, recommend that if you have a fuel burning furnace, stove or fireplace, they be inspected for proper functioning and serviced before each heating season to protect against carbon monoxide poisoning.