Nothing. Any more than wiping off the case exterior or tumbling in 20/40 grit corn media for 30 minutes, you are simply working on your ego and not making your reloads any better.
Well yes I guess you're right, its my ego. Same reason I once drove a New Corvette, same reason I never leave the house unless I'm shaved and dressed in clean pressed cloths, keeping my hair combed, and looking a little more than just presentable. Keeping my shoes shined. Dressed in a pressed sharp looking Uniform for Military Funeral detail when required. Keeping my firearms well oiled and cases spotless and sharp looking, and the same with all my shooting gear. Keeping my vehicles waxed and sweept out and clean without an inch of mud and sand on the floor.
Yup its all my ego!
Oh, and I might mention PRIDE sorta fits in here also.
Brasso. It won't hurt or weaken your brass. Just another internet rumor. It's the only thing I've used in my corncob media for 20+ years. Think about it..it's called BRASSo...formulated to clean BRASS.
Liquid Brasso WILL weaken your brass as it contains ammonia. Believe it if you wish, don't if you do not wish to, makes no difference to me. However once the ammonia evaporates out of the mis its no longer present and being as that is the case it no longer presents a problem of course. If in fact you are in reality a 4-H reloading instructor I would suggest you do some research regarding ammonia and the degradation of brass.
..it's called BRASSo...formulated to clean BRASS.
Yup, thats what its called, but nowhere does it claim to be used for cleaning brass ammo cases. For your reading pleasure
http://en.wikipedia.org/wiki/Brasso
You clean your brass before you size it? Never heard of that. How do you get the case lube off after sizing? Clean again? Just curious.
If handgun brass and not bottlenecked there is no reason to lube the cases if using carbide, however I usually do with a spray of teflon spray using "Blaster" brand, no reason to remove it as its a dry lube.
If rifle cases I generally lube with Unique case lube or Mink Oil, then rinse in white gas, yes its flamable, but I do it outside and its no more dangerous than filling my lawnmower. Then prime and finish loading. If I want really sharp looking rounds I tumble the finished loaded rounds for a final polish.
Hope this helps and clears a few things up!