I agree completely with herman. As I recall, Sherwood has a 2 year warranty, so at most your cost will be about $20.00 (service kits are about $10.00 ea. for both stages) for parts over this 2 year period. The major cost is for the needless service. I dived Sherwoods extensively in the '90's, and, unless there was a minor adjustment needed, I would not even think about servicing them until at least 300 dives. Both seats in a Magnum are simple pucks and can be, and often are simply flipped to use the other side by individuals who service their own Sherwoods.
My g/f is using an Oasis's, and I recently flipped the first stage teflon seat after about 250 dives. It was not really needed, as the IP was solid, but I happened to be in the area because I needed to replace the dry bleed orifice filter, which had become clogged.
Sherwood scuba Rep. is giving you the company line as he is required to do. This is essentially the same approach you will hear from every regulator company.
I recently met a couple in the Caymans who were diving older Sherwood regs. They told me they take 1 or 2 trips a year (about 30 dives), and have since the early 80's. The man's reg developed a slight freeflow during this trip and he had it serviced by the resort shop--had it back in about an hour. Curious, I asked him how long it had been since the last service. He said, "well we got them used in 1987, and they have not been serviced since". Wow.