I would say that's a matter of personal preference. Some folks brown it, some blue it, some polish it and leave it bare ("armory bright"), some use bleach or other things to give it a patina, it depends on what YOU want the gun to look like.
The locks we get are investment castings and the finish is "unfinished" and unlike anything appearing on originals. It's just too labor-intensive for lockmakers to polish them properly and locks would cost 2x as much as they do now. All gunlocks on originals were well polished, then either left bright, blued or browned as indicated above. I've always filed and polished them.