Out to three hundred yards, I have shot mixed cases with excellent results. You can load ammo with lots of run out, +- half grain (maybe more) variations in powder (30 caliber), seating depths all over the place, an absolutely random selection of cases, and still clean the NRA target at 300 yards.
At 600 yards and further, I shoot only one brand of good brass at a time. (W/W, Rem, LC, IMI). And I am much more careful about my loads.
When I become rich and famous I will be able to afford Norma and Lapua brass.
I stay away from Eastern block brass, the stuff I have run across has not been very good.