I used to do that before I got a tumbler and it works fine though the brass tarnishes a little pretty quickly. NRA published that method long ago so it has a good pedigree.

May I suggest your tumbler technique isn't right? I use corn cob with a little brass polish (non-ammonia type!) and follow with clean corn cob to get off the little bit of film the polish leaves. Cleans better and faster if you consider drying time and the brass doesn't tarnish as quickly. Even warmed in an oven it took overnight to make sure the inside of the case was bone dry. Of course that little bit of tarnish means nothing except for looks.

I bought a bottle of Midway brand brass polish at a gun show years ago and may never use it up. A little goes a long way.

The key elements in human thinking are not numbers but labels of fuzzy sets. -- L. Zadeh

Which explains a lot.