JTM - if you're looking for votes, I vote that you leave the brass the way it is. To me, it looks more authentic that way, especially with the rest of the metal, rust excepted. Heck, I regretted polishing the brass receiver on my 1866 replica Navy Arms lever-action, but had plenty of time to let it get back to tarnished.
I don't know, and haven't read, if cowboys polished the brass on their guns in the 1800s, but I wouldn't think there was any advantage to doing so. If soldiers did, it probably was only because it was required.
Nice-looking gun, BTW. Hope you have a lot of fun with it. In the good-humor vein, you might get enough lead out of the barrel to make a bullet, or else you could try being a pioneer with black-powder snake-shot.