Rolex used to replace all worn parts in their movements as part of their servicing at no extra charge. Do they still do this? If so, then I would push off the service as long as possible. If you wear out a bunch of parts after 20 years they'll be replaced in a single service. I think the average service interval for them is 10 years (not that they recommend that but what people usually do) and the servicing is like $750 last I remember.