For the casual shooter ( read here old eyes and shaky hands) is it really critical to keep all your headstamps the same for any given load? In my experience , be it ever so limited , I cant really say that I can notice a difference.

yes, sir, its critical

brass can vary 3-5% in capacity and thickness ..

case in point, my most accurate rifle, a 257 roberts ... it taught me an easy way to prove ..
with remington, winchester+p and frontier (hornady) brass, it shoots 3 distincely different, but great groups.

if you load 5 each of these, and shoot all 15 at the same point of aim, it will result in THREE seperate groups .. each group is great, but are inches apart ...

do try to keep your brass seperated . at least for semi accurate guns.

for blasting ammo, or for me to knock out 500 pieces of 40cal, I don't care about headstamps .. my wife and her friends shoot her 10 and their 40s (yes, hers cycles the 40s just fine) at 20 yards, and eat up a 6" bull ... and the brass is so cheap, for 1x, its not worth seperating

Amen!!

Amen with some provisos. Sometimes two different brands of brass will shoot to the same place in which case you can use them interchangeably. You will only know this by personal testing.

Also, some brass with different head stamps is produced by the same manufacturer. Norma, for instance also produces brass with the Weatherby head stamp.