After rebuilding a couple of old bikes, I think each person develops a certain "style" to how they rebuild bikes. Some people prefer to modify bikes, some restore with correct or period parts, and some do only light preservation. Some people mix all three methods. Some people have a style that changes over time, based on the circumstances of each bike.
If I had to describe my preferred style today, it would be a mixture of "practical", "correct" and "time period". The idea is to maintain the history of the bike while also making is useful in the way it was originally intended to be used. My bikes are usually comfortable, practical machines as was originally intended by the manufacturer when they were made many years ago.
"Use original parts when possible, but if need be, replace them. The best replacement is a direct original. Second choice would be a period correct part. Third choice would be a new part that is similar to the original. Last resort is a new part that is quite different from the original."
I preserve original parts wherever possible and consistent with the safety and use of the bike. Even if an original part is worn, it should be used when doing so is still consistent with safety and function. Is there a compelling reason to remove an original part? If not, try to keep using it. Sometimes there is a safety or function reason to swap a part, but try to use originals when you can.
A direct, correct part is the best replacement when you have to replace an original. Sometimes this is possible and sometimes not. Go with a correct/direct replacement if you can.
If you can't get a correct/direct replacement, try a "period correct" part, a replacement part from the same time period.
If a period correct part is also not available, try a newer part that matches the technology and style of the bike. For example, a new Brooks B66 saddle on a three speed bike is perfectly acceptable if the original B66 is dried or broken. Parts made of leather or rubber often degrade over time. Or for example, use a new slotted screw to replace an old slotted screw that has stripped or broken. Using a hex headed screw on a 1940 Schwinn or Raleigh would look out of place, but a new slotted screw is closer.
"A vintage bicycle should be reasonably safe and useful. It was intended to be ridden and your project should function consistent with that intent. Don't be afraid to replace consumable parts that affect safety and ride."
I replace consumable parts that have an effect on ride and safety. I replace rubber brake blocks/pads, worn brake cables, worn/damaged cable housings, worn shifter cables, worn tires/tubes, damaged handlebar grips, etc. It's not useful to have original brake blocks that squeal and won't stop the bike, or cables that are original but snap when you need to stop quickly, or tires that frequently flat because the sidewalls blow. I suppose this would not apply to "display only" bikes in museums and the like. But for my purposes, the bike needs to work and be reasonably safe. New parts should be similar in appearance and function to the old ones, where possible.
"Remove grime, dirt, and rust wherever possible. Do not remove good, old paint - even if it is faded. Gently clean the old paint and decals."
I remove rust and clean off dirt/grime. Rust is simply a chemical reaction that corrodes ferrous metals such as iron and steel. The science is fairly straightforward. Rust should not be called "patina", which is often what a salesman will try to call it ["patina" originally referred to the green-ish oxidation of copper or bronze metals, something different]. I remove rust to the extent I can do so because it is a degradation of metal and should be cleaned up. I match and patch-up paint to prevent rust from returning. I treat bare metal or damaged chrome with WD-40 and similar products to help stave off rust.
I clean up and lightly polish old paint. Faded paint helps give the old bike its character. I gently clean and polish it, but I do not remove it or try to make it look brand new. I gently clean up old decals and leave them in place whenever possible.
"Check all mechanical parts for damage or wear. Replace broken parts, bad bearings, bad races etc. Everything should work smoothly and as-intended by the manufacturer."
The bike should work properly. Wheels should turn smoothly and run true. Steering should be smooth, reliable, and precise. Brakes should feel crisp and grab reliably (within the limitations of their engineering). A bike that looks nice may not function well if you don't clean and check all the mechanical items.
"Add accessories that make the bike safe. Observe safety laws. Try to use accessories that capture the look of the bike but which also add an element of safety."
If you need add-on LED lights or a bag with a reflector on it to be safe interacting with traffic, do it. I like retro lights that look correct from the outside, but which contain modern LEDs inside to aid visibility. I also like LED conversions for period lights, such as LED lights for Dynohub lamps.
