So when is it not okay to take an old tool and restore it? Ya know, like take away the patina its taken years to acquire.
That's a personal opinion sort of thing.
Personally, I prefer seeing the mark of years of use on an old tool. If it's been heavily used, I like seeing the proof of that; wear in places where it was gripped, discoloration from the oils of a user's hands, even the occasional nick or hammer mark where someone wasn't quite careful enough. That said, work surfaces need to be correct. I had no problem dropping a wooden plane in my vice and truing the bottom with a try plane, or grinding a new bevel on an old chisel.
So,
in my opinion, there's a difference between cosmetic and functional restoration. Cosmetic restoration is only ever reasonable if the tool is messed up to the point of "I can't use that, I can't even bear to LOOK at that!" Functional restoration is always acceptable unless the point is to have a museum piece that will never be used. I make an exception for metal tools that need a finish (like japanning) replaced to prevent rust.
Other people have other opinions, ranging from "Old tools should be preserved exactly as they are, without even functional repairs" to "pass the wire brush, I'm gonna make this thing look factory new!"