I don't really have rules, but mostly I don't see the point. I've been riding long enough to know that vintage parts worked just fine BITD, and they still do. Part of the reason I got into cycling is that I enjoyed bikes as little works of industrial sculpture. To my eyes, putting a bunch of modern parts on say a 70s racing bike just looks bad. The presumption by many people seems to be that modern parts are better, but really they are not going to make you faster, realistically. It seems to me that mostly it is a matter of making old bikes more familiar to people who have become accustomed to brifters and index shifting.

If I want to ride a modern bike, I'll ride a modern bike.

That said, I'm not religious about it. For a practical rider, switching in clipless pedals is very reasonable, mostly because you can no longer buy old school cycling shoes that were made for toe clips and straps. Frankly, they used to bother me BITD. I don't really need to relive straps cutting into my feet, literally, on hard steep climbs. For that matter I don't have any great urge to relive doing Eroica type rides, been there and did that for real on friction shift old steel and sewups.

Other components I look at on a case by case basis. Sometimes, if you start from a bare frame, it just doesn't make sense economically to put the "correct" parts back on. In this case, I'd rather put modern retro parts on there, but if someone else prefers to put a full modern group on there, nothing really wrong with that, especially if it encourages them ride the bike more. What I don't like is when people take off the vintage parts and replace them with modern, to ostensibly upgrade the bike.

It sort of depends on the era as well. I'm not really that enamored with any post Tullio Campy, and it doesn't bother me one iota to see racing bikes from the late 80s onward modernized.