Some observations from having maintained various boilers around the family for years...
I would say that any open flued boiler should be serviced every 12 months because the risks from poor combustion and fume leakage are significant and easily lethal. (We don't have any of these for some years now.)
This would be followed by boilers which are room sealed but not fanned flues and still rely on flue gas convection for correct burning.
IME, fanned flue boilers do a much better job of staying clean and burning well (whether condensing or older). If you have a flue gas analyser, you can easily check their combustion without opening them up.
Potterton Profile (light use) and Potterton Suprima (heavy use) showed no deterioration in CO/CO2 ratio, although I decided to open and service after 7 years. In both cases, nothing actually needed doing except vacuuming out a layer of dried flies/spiders which wasn't impacting the operation, although I did blow any dust out of jets/burners too. Heat exchangers have remained clean.
Keston condensing - initially did this annually, but Keston told me that tends to do more harm than good. Top burner gasket lasts about
4 years, so it now gets done that often, plus an annual flue gas check. Also flushing debris out of the bottom of the combustion chamber once or twice a year, after experience showed it could block the condensate drain otherwise (this can be done without opening it up). Regularly check for condensate leaks, which are well-known for writing off these boilers if not caught quickly.Main multipoint water heater - needs dust blowing out of jets and mixing tubes every few years. Older ones need descaling - newer one with teflon coated heat exchanger tubes has never needed descaling.