Well, I think Flitz is slightly abrasive, so I do use it sparingly.
The handle is stainless. Brass will always darken a little, even in a presentation case. A little Flitz or other metal polish
will bring it back easily.
If it's a regular user ... don't sweat it. It will get a little "character" over the years and be fine. As long as it's "used and not abused."
Tony