Never a good idea, and here is why.
When air supply and/or air return ducts are present in a room or space, the detector(s) should not be placed in the path of the air flow supply or return (NFPA 72-2007). Placement of detectors near air conditioning or incoming air vents can cause excessive accumulation of dust and dirt on the detectors. This dirt can cause detectors to malfunction and cause unwanted alarms. Detectors should not be located closer than 3 feet from an air supply diffuser or an air return vent. This a a recommendation, not a requirement for a transfer of property.
The same reasoning for ceiling fans in bedrooms. Dust and dirt is attracted to ceiling fans and will destroy smoke detectors over time. Place smoke detectors at least 36 inches away from ceiling fans in bedrooms.
Questions or comments regarding where to install smoke detectors when selling in Southern California? Ask away in the comment section below.