My process is to take the bicycle apart, de-rust, clean and re-lube, address any mechanical issues, polish, and re-assemble. Preservation is a good way of looking at this. But it's also preservation with a focus on mechanical integrity. I ride 4-5 nights weekly, spring through fall, over hilly terrain on public roads at speeds from a crawl up to about 30 mph. I'm not racing, but I am asking the bike to do what it was designed to do in terms of general purpose riding. So it has to be a presentable old bike, but it needs to work and be safe. I don't ride through road salt or in the winter.
If a bike requires a total re-paint and re-chrome, I don't buy into the project. Nothing I own is valuable enough to justify that anyway. And the level to which you do all this work is entirely a case-by-case basis. Each project is its own case and you need to study what you have before you act on it. A highly valuable and complete project that is in bad cosmetic state might be a candidate for a full concourse restoration, whereas a $200 bicycle that has aged but original paint is worth just cleaning up. "Do no harm" is a good way of summing up the most basic rule of all this.