I will also say that it will lower the value to other collectors.
I don’t collect locks to later sell. If I get a lock that is “IN” my collection I will not let it go so I will clean and redo the finish of the lock as close to the original as I can. In fact I am working on one now that I will be doing an oil rubbed brass Unit lock. I am planning on using Metal Darkening Solution from Architecturals.net (Yes I will post before and after pictures, but I have just started the project.) I have completed a cast iron and bronze lock that I have posted pictures here and I am building mounts for that and the lock I am working on. The only thing that I try to keep in mind when doing this is that I want a lock that would look “New” and work like new too. Sourcing the key blanks is the only hard part as I want them to have the OEM logo of that time. I like having show quality locks in my collection instead of that scratched lock that looks like it should be scrapped. 8-)
http://www.uniquelighting.com/Spec%20Sheets/PatinaTreatment.pdf
Here is a DYI on Patina that I tripped over.