Before beginning my first batch of .223 loads, how much difference does it really make when using different brands of cases?
Not much difference unless you load a near max powder charge.
Keep the charge weight in the low to mid range for the powder and bullet you are using and you will not have a problem. Couple times a year i load a big batch of rounds using mixed cases for use in my AR-15s.
If you want accurate loads for hunting use just one brand of brass.
For my accuracy bragging loads i weigh my cases and segregate them by weight. The cases are then prepped by trimming, uniforming the primer pockets, and de-burring the flash holes. These loads are gun specific. Sometimes i wonder if all this effort is worth a 1/8-1/4 inch tighter group.