Dust on a return grille doesn't, by itself, mean you need the ducts cleaned. EPA recommends duct cleaning as needed rather than as a routine service.
Before buying it, ask the company to show what it found, where it found it and why cleaning is the appropriate work.
Its guidance identifies conditions to consider, including substantial visible mold in hard surface ducts or other system components, vermin infestation, and excessive dust or debris clogging ducts or being released into the home.
Suspected mold may need expert confirmation. Ask for the location and context of any photos. A close up without a clear connection to your system leaves important questions unanswered.
Also ask which portions were inspected. Some ducts or components may not have been visible.
Cleaning addresses contamination. Sealing addresses leaks in the duct system. If the complaint is a cold bedroom or poor cooling, the company needs to explain how its proposed work addresses that complaint.
Dusty grilles and an air delivery problem can appear in the same home without being the same problem. Don't assume one cleaning package resolves both.
Ask whether the recommendation includes repairs, sealing or correction of a moisture source, and how those tasks differ from cleaning.
Get the included ducts and system components listed, along with access work and how the company documents completion. “Clean the vents” leaves much of the system unspecified.
If the company proposes a chemical treatment or internal coating, ask for the product and purpose. EPA discusses limits and concerns around these treatments; read that guidance before accepting one as a routine part of the job.
EPA says duct cleaning hasn't been shown to prevent health problems. A sales claim that cleaning will solve household symptoms goes beyond that evidence. Discuss those concerns with an appropriate health professional.
The useful decision is about the condition documented in your system and the work proposed to address it. If the concern is air delivery rather than contamination, our HVAC sizing guide explains why ducts belong in the assessment alongside equipment.