I think a lot depends on what you are trying to do.
I know what the purists are saying, that you destroy the original specimen, but, just one little thing: guns are made to shoot. I think that if you can scope the critter without permanently destroying the original markings, that is, doing it in such a way that it CAN be put back to original, you're not destroying the rifle.
There is this neat little invention called a "plug screw", you know. And there are very small files and there is Birchwood Casey paste blue.....
Perhaps not original.... but how ELSE are you going to really understand the potential of the equipment you are collecting. A friend has one of those Norwegian .30-'06 Mark 98k rifles. Very pretty. Trouble was that no matter what he amed at, he hit. So he had a very small bit removed from the wood, got another bolt and had it bent.... and he scoped the critter. We now know just HOW accurate those rifles could actually be.
And how accurate can one be?
One third of a minute, any range.
You CANNOT do that kind of testing with iron sights.
And he has plug-screws, too.