I just let it tarnish--that's what brass does fer a livin'. I like my stuff natural. But, the old GI in me still polishes the brass when it needs it, and sometimes, even if it don't. I have a motorized buffer in my shop and with the right rouge, it only takes a second or two and gives me a reason to fondle my knives. Over the years, I've found lots of orphan Bucks, that are grimy and dirty, with or without leather, and could pick them up pretty reasonably. Clean'm up, polish'm up and make new leather and presto, you're back in business. I have a hard wheel on one side of my buffer for doing the scratches in the steel and a soft wheel for doing the brass/aluminum/and phenolic handles.
IMPORTANT NOTE: if you use a motorized buffing machine like mine to do the steel blades, don't try to rush the job and press too hard. The wheel will grab the blade and you won't like the results. It gets blood all over the work bench.
Jack