Some of the early seat materials weren't that durable, so a one year service interval was reasonable. Or at least not unreasonable. And it kind of made sense to look over everything before the start of the season for people who lived where diving was only done in part of the year.
As materials have gotten better and dive travel means there's no set season for most divers, divers started feeling that an annual service was unnecessary. Eventually the major manufacturers responded and have moved to 2 year intervals.
But if you want to step off the scheduled bus, you can switch to servicing your gear based on condition. You just need an IP gauge, a tank and a willingness to occasionally check how everything is functioning.
Discussion of this thread can be found here: http://www.scubaboard.com/forums/regulators/260452-regulator-checklist-inspection.html First off, I would like to acknowledge my friends who have done more than their share in developing this checklist. Due to my laziness and resistance to answering...
scubaboard.com