It really depends on what you want in your reloads. I sort my brass by head stamps when I'm loading what I consider to be good bullets. Hornady, Sierra, Speer ect. Sometimes I find bulk 250 packs of fmj and when I do I just load them up in mixed brass and go play.
What he said... Mostly, if you are looking for bulk ammo reloading just for fun and aren't overly concerned with tiny groups a safe way to accomplish using mixed brass:
1-Clean all brass.
2-Size all brass.
3-Inspect and measure all brass and cull the bad ones.
4-Trim them all to uniform length.
5-carefully work up a load that is a bit beyond mid-power (say 2/10 grain).
6-Do your bulk loading at the mid power level, this provides room for error.
7- measure all fired brass. Different head stamps will not stretch uniformly. I keep the ones that are "close enough" and cull those that stretch noticeably more than the average.
8- repeat (you can skip trimming until brass it close to max length).
Note: step 6 doesn't mean you don't have to check your powder throws every so often. It just gives a margin for error.
Keep in mind that this is the system I use when I'm shooting several hundred rounds in a session and I don't expect tiny groups.