I still go by the advice from the old NRA book from the 1970's, that regular mottling from general use and age, NOT GRIME, actually protects the brass from further degradation. I clean actual foreign matter from the outside gently, but I've never tumbled a piece of brass. Considering what I've loaded has worked, I'm not sure how much there is to gain from tumbling, and I'm not too interested either. I've heard many a board member here say that tumbling is actually one of the most dangerous lead raising things you can do, outside of shooting lead containing primers indoors. I sometimes think about taking it up, but....
I suppose one thing going for me is shooting on my own property, and being extremely careful with my brass. I try to pick up every piece I intend to reload every time I'm finished with a set, cleaning up often, to keep any from settling into dirt or getting dirty. I'll often times feed my bolt actions slowly, one round at a time without the magazine, to keep each individual piece from flying around. Revolvers and moon clips are easy, and even for drills I'll stand on a blanket to catch my casings carefully. Expensive 10 gauge hulls are tracked carefully, none must go to waste. When I do get dirty brass, I tend to segregate it, leading it to live a lonely existence in a box I'll eventually "get to", for eternity.