My thoughts, based largely on my local climate, is the following:
- Natural gas and propane fired furnaces should be inspected and serviced yearly. This includes heat exchanger inspection, venting safety inspection, and combustion analysis, not a $29.95 "special"
- Heat pump and regular a/c outdoor coils should be cleaned at the beginning of the cooling season, every year
- Heat pump and straight a/c outdoor compressor contactors, terminal connections, wiring, and capacitors, outdoor fan motor, blades, and capacitors, and all wiring associated with all components require annual inspection and testing, at minimum
- For cheap 1" filters, homeowner should change them every 30 to 45 days
- For a deep pleated (4") media filter, homeowner should inspect it every three months, consider changing it after six months, and definitely not let it go more than one year without a change
- Refrigerant level should be checked only if a leak is suspected, or performance deficiencies have cropped up, provided the technician or company is already familiar with the system's history. A new tech/contractor to the system should check levels to establish a baseline comparison for future visits. Leaks should always be repaired; "topping off" a system places system longevity at risk (i.e. potential compressor death)