I agree that the need for service can vary. For example I dive a lot and when I used to do 130 or so dives per year with a single reg, it was not uncommon to encounter problems with IP creep, especially with the Mk 10 Plus and Mk 15 with their early concave seats and seat material (the current MK 20/25 seats and material used in these seats as well as new Mk 10 Plus and Mk 15 seats are much better). Once I switched to doubles and started swapping primary and backup regs mid season to even out the use, going a full year between annual services was not a problem (130 dives total per reg but only 60-65 dives per reg where the reg sees numerous inhalation cycles.)
The point is that a lightly used reg with one or two tropical trips with proper rinsing and care since the last annual service does not really need a full annual service. A flow test, a new low pressure seat in the second stage are more than enough in most cases. In contrast, a reg that has seen 100 or more dives or 50 or more hours of operation under water, probably would benefit from a full rebuild even if 12 months have not yet expired as it is beyond amount of use where you can expect continued problem free operation. In fact many shops with heavily used rental or pool regs will service them 2 or 3 times per year to keep them within thes general time between service limits.
I also agree that I will at times I have taken liberties with my regs that I will not consider taking with a customer reg. I have an impressive save a dive kit and have on one or two occassions done a 10 minute mini-rebuild of amy reg on the boat when I had a pre-dive check problem that would have caused a customer to miss a dive.
I should clarify that I replace dynamic o-rings (piston head, piston stem, seat carrier, etc) without question at each rebuild. With semi-dynamic o-rings like the swivel cap o-ring, usually not as they are larger, under low stress, are easy to remove with no risk of damage and are usually in very good to excellent condition even after years of use. With smaller semi-dynamic ones like the one in LP hose swivel, they usually get replaced as they see more rotational movement, more dirt and grime, are impossible to inspect without removing them, and removing them has significant potential to damage them through excessive stretching. In very rare cases where I do not have a replacement for that particular o-ring, the "it's not broke so don't fix it" dictum applies and I leave it entirely alone as removing and inspecting does more damage and can render it unserviceable.
Static o-rings like the HP and LP port o-rings get replaced if they are flattened, brittle/stiff or show signs of damage such as nicks or feathers. If they are fine they get relubed and reinstalled. If there is any doubt or if they have obviously been in service for years, they get replaced anyway.
