None of my bikes are historically significant or one-of-kind rare. Most are iconic and difficult to procure, but that doesn't make them museum pieces. My theory of repair and refurbishment is as follows: Most good frames receive new components after the original component saw a few years of serious use. Replacing components with a more modern generation is acceptable.
Almost every bike I touch has a few components that would not have been available at the time the bike frame was completed. I like open tubular tires and only one of my bike has sew-ups with period correct rims. I also find it tempting to add a few cogs to the gear range and Ergo levers in place of downtube friction shifters. These changes allow me to ride smoothly in a moderately fast group. A bike should function and perform very well, but should retain a classic aesthetic.
However, I won't randomly add modern components. I like to think any modernization is done with respect for the heritage of the bike. I usually replace Campagnolo components with Campagnolo components that are newer, but not modern, for example. I'll have the paint expertly spot repaired, but I won't do complete repaints.
Here is an example: 2x9 Campagnola Chorus drivetrain, shifters and brakes. Chorus hubs with Velocity A23 rims. 700x28 Vittorio Corsa G+ tires. NOS S. Marco Concor genuine suede saddle.
What are your rules?