It depends on the person.
Once you've shot a match, and you get a bit "crazy, anal, whatever...." about your brass, and brass prep, I guess you becomed ruined about how you treat your brass, and can't lump it all together anymore.
Its not just separated by BRAND, its LOT that is iportant.
On Factory Loaded ammo, Lot numbers are on back of the box. If you purchase ammo in larger sums, you can look at the back of the box, and match Lot numbers, that way you will be ensured that your ammo will place the same on target.
This may be a bit that folks didn't know, may even confuse a few, but for those that shoot precision rifle, they already knew it, or maybe it will help some in a small way.