Agreeing with snake-eyes, I think I'll emphasize the word "collector".
IMO, there are old guns and collector guns.
An old gun is obviously one that is old, but more importantly it was made by someone that no one ever heard of.
Unless it has some interesting feature or superb workmanship, collectors aren't really very interested in these, and at least to me, the idea of "fixing it up and getting it in shooting condition" is a worthy goal.
On the other hand, a gun made by a known gunsmith or company like Ketland, Dickert, Amos, Hawken etc falls into the type that collectors are interested in and probably should be left as they are. Of course, cleaning them up using mild detergents which won't harm the existing finish is OK but replacing hardware can affect the value of the piece.
Speaking of antique furniture, every year dozens of pieces are damaged by people who want to refinish them. In doing so they can decrease the value over 75 percent. Collector guns are a lot like old furniture.
zonie