For one, the diameter of sized brass will change when you seat a bullet. Plus there is always some variation in the thickness of the brass from different manufacturers, sometimes a really big difference. When people are chasing that last bit of accuracy they will usually stay with one brand of brass just for this reason. If you're not after that last possible .0001" in accuracy it probably won't matter to you. The thicker brass will change your neck tension, which if varied a lot, will have some effect on accuracy AND pressure. If you're loading any maximum loads, stick with one brand of brass. If you change brands, you might need to work up the load again, watching for pressure signs.
Cheers,
crkckr