I go both ways on the issue.
Regs are for the most part simple and usually trouble free and most failures are of the slow leak variety and occasionally an undertrained tech is in fact the biggest maintence threat you face.
But, service needs differ as while a little used and well stored and maintained reg may do just fine after 3-4 years wthout service, I have seen regs serviced 3-4 months ago that stopped working due to being flooded in a rinse bucket on the last dive trip.
And finally, this weekend, 1000' back in a cave, 90 feet deep and on a deco dive to boot, I really appreciated being able to be fully confident about the maintenence status of my regs as despite having two of them, it was still not a situation where I wanted to have to deal with a reg failure.
So it depends on you, it depends on your diving, it depnds on your shop and it depends on the type of diving you do. But if there was any doubt at all, I'd have them serviced annually and not sweat the cost. Compared to the other expenses of diving over the course of a year the annual service cost is cheap even if you pay for the parts.