I don't think there is a rule that applies to all. Obviously it is easier to follow just one rule instead of analyzing each particular case.
If the scenario is a number of regs that get rented out, I'd like to think the people in charge (owners?) keep a maintenance log of each unit and do a complete service once a year.
If it is a diver that uses the regs a couple of weeks a year, then stores them in the garage for the other 50 weeks of the year, and doesn't do his own service I suppose it wouldn't hurt to service them once a year to make sure nothing happened to it while stored in the garage, (moisture, mold, ants, gremlins?)
Now for people that dive every week and service their own units. I see no reasson to service each and every unit when the 365 day count is up. I personally dislike the change after a service, yes the unit breathes correctly but not exactly the way it did before the service, it takes me 4 or 5 dives to get over it. To me is like changing the band in the mask, I have to change it every 4 or 5 years and is just not the same.
So, you keep how ever many service kits you need at home, that way when any of your units start acting funny you go ahead and service them. This past month it happened to one of my units, checking dates it turned out to be over 3 years since the last service. The unit could've been used longer but I felt it was time. After opening it and going thru the process my husband decided he'll service the unit he's using these days too. His decission was not based on dates or on the unit behaviour, he just thought he has the time these days and has been long enough since the previous service.
So the answer in my household is "depends"