I load 100 rounds at a time and always start out with 100 rounds of the same headstamp. I gather my brass as I shoot and look to find all of that box. If I lose any I replace with the same headstamp from spares. I also keep stickers on my ammo boxes showing the data for each of the times reloaded. The number of reloads may range from new to 20/30 times. One reason I don't mix.
My 38 Supers all started life as new Starline but were put into production over a period of time, so a varying number of reloads.
I just acquired 4K of once fired 45 ACP small primer brass, half Speer and half Federal NT. Since they will all go into production about the same time I may not be so pickey with them--but I may. I am having to swage the primer pockets of the Federal, so the definitely load different than the Speer.
You have to keep in mind though that I have been a CPA for 50 years, and am still trying to learn how to pass my days without working.