This is a question that may cause me a lot of grief personally, I have every thought that someone who runs a duct cleaning service will get together with the "duct cleaners of America" and show up under my balcony with torches. So let me say that I live to learn and will research further anywhere you tell me to look, except in your sales literature of course. I think I'll pause for a cup of tea from that kettle on my stove (I am the writer and the photographer on this website). My observation over the last 30+ years has given me an opinion on this duct cleaning issue. First there is no harm in cleaning ducts, there's no harm in cleaning anything in your home. Unless the insulation is damaged, or the seal removed from the face of the insulation with solvents. 
 But it's like this, whatever household dirt or dust may have accumulated over the years is likely to stay where it is. When Rainbow of Ohio replaced furnaces in Century (over 100 yr.) homes, we were faced with the problem that any renovation of the structure could not show, or the home would lose it's status of historical home. So no package unit sitting outside with all new ducts was considered. The existing coal furnace ducts had to be reused where possible, and let me tell you when we fired up the new equipment (even with moist cheesecloth over the registers) what was in there came out. 
 Then an equilibrium was reached, and whatever might have been left, stayed, without any adverse effect we could measure. Nothing has changed. If you are considering a new furnace and the airflow will be significantly increased, by all means have a cleaning in advance. If you have had a fire, or as part of mold remediation, or even obvious debris in the ducts, then cleaning is appropriate, and recommended.
 I do not stand alone on this, so let me quote the EPA from their documents unedited.
 Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space. It is important to keep in mind that dirty air ducts are only one of many possible sources of particles that are present in homes. Pollutants that enter the home both from outdoors and indoor activities such as cooking, cleaning, smoking, or just moving around can cause greater exposure to contaminants than dirty air ducts. Moreover, there is no evidence that a light amount of household dust or other particulate mater in air ducts poses any risk to your health.

 EPA does not recommend that air ducts be cleaned except on an as-needed basis because of the continuing uncertainty about the benefits of duct cleaning under most circumstances. 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. Some research also suggests that cleaning dirty cooling coils, fans and heat exchangers can improve the efficiency of heating and cooling systems. However, little evidence exists to indicate that simply cleaning the duct system will increase your system's efficiency.

 Some research suggests that cleaning heating and cooling system components (e.g., cooling coils, fans and heat exchangers) may improve the efficiency of your system, resulting in a longer operating life, as well as some energy and maintenance cost savings. However, little evidence exists that cleaning only the ducts will improve the efficiency of the system.

Nuff Said..EMC