Really, it's an economic decision you have to make for yourself. Here's some info.
The labor (not a "fee") for an overhaul (replacing wear-outable and dry-outable parts) will range from $15 to $25 per stage. Labor for a reg "check" will run you $15 to $30 for a reg set (free if you're a *really* good customer).
For SP regs, if you do not have the parts warrenty, parts will run you retail $20 stage (first stages), $10-15 (second stages). (All figures are ballpark and from memory).
FWIW, I have one SP reg set, and I have a complete overhaul performed every year. At bare minimum, I would recommened the reg being checked every year, and overhauled every other. Keep in mind that it's not just useage that wears out internal parts. O-rings and rubber break down over time, seats take a set, parts need to be re-lubed and spiders dug out!
Also, I am under the impression that the SP parts warrenty also covers any mandatory upgrades SP orders in the future. It also covers parts that wear out even if these are not "annual service kit" parts. i.e. the crown to your HP seat gets worn and is leaking, or the spring for the piston shows stress fractures, the parts warrenty will cover the replacement of these parts (everything but labor charges). W/O the parts warrenty, you pay retail for those replacement parts *above* the cost of the annual service kit. Those could be $20 to $60 parts down the road.
In short, my advice would be: it's life support equipment, don't be a nickel rocket and skimp on the annual overhaul. Take it to a technician with good credentials and lots of experience, not the cheapest. Keep the warrenty up. YMMV.
