First, for most SP regs, there is very little that actually NEEDS replaced.
For BP 1sts, you have the seat (which should only be tampered with if you have IP creep) and a couple of dynamic O-rings (the piston LP seal and the case HP seal) which MUST be replaced when you tear it down (according to any good O-ring practice - any dynamic seal that has been pressurized gets changed)
Assuming the seat and piston are OK (no IP creep) that handful of O-rings (literally two to four, depending on the model in question) is worth PERHAPS a buck.
Realize that a BP first has NO "set" on the seat that is taken over time, as the piston is NOT against the seat unless the reg is under pressure! Absent getting crud in there the only "damage" you should see to the soft parts when not in use would be on the dynamic O-rings.
The second should probably have the seat replaced annually for non-balanced poppet designs - the "second overhaul kits" are $10 at retail, and they contain a lot of things that probably don't really need to be changed (but "should" be according to SP)
I just picked up an older MK10 for dedicated use on my O2 analyzer. It had pretty nasty IP creep. I put a new seat in it, cleaned it generally, replaced the dynamic O-rings and put it back together.
It really could use a new piston, as the knife-edge isn't quite as sharp as it should be, so it still creeps a bit - but for what I'm using it for, it'll be fine (if I intended to dive it I'd probably get a new piston for it. I may take a lapping block to the piston and see if I can clean it up, but only if I have a spare piston already around, 'cause there is a risk of trashing the seal entirely by doing that.)
I "burned" a Mk10 rebuild kit doing it - oh darn, there goes $10 - but some of the parts in the kit did not need replacement, so I still have 'em, so did I really burn the "whole" $10? Nope...
I would tear down a working BP 1st (no ip creep, good response on inhalation, etc) to change the dynamic O-rings annually, inspect for internal corrosion, and clean the ambient chamber, but that is the only reason to do it. If you have creep then you need to replace either the seat, piston or both. If you have corrosion then you need to take care of it and change whatever bad habits you have that are allowing it to happen. Otherwise leave the damn thing alone!
Seconds are a bit tricker; the cover ought to come off annually anyway, if for no reason other than to check the diaphram and exhaust valve condition. If it won't hold adjustment then tear it down and rebuild it. But if it does and is breathing ok, and shows no signs of corrosion internally when you take the cover off, why are you tearing it down? Because some shop monkey told you to? Is that really smart?
Realize that an awful lot of people run into trouble IMMEDIATELY after having something "serviced." Why? Well, if you're paying $30 for a stage in labor, exactly how much care are your regs getting from the tech? How careful is he/she being?
This is unfortunate, but true.
Regs aren't rocket science, but if they're working ok there is a good debate to be had as to whether you ought to fix that which is not broken.
