Okay, let a grumpy ol' f**t reminisice ...
Years ago, the late Skeeter Skelton wrote a column about "Cow Killer Loads."
Seems that he used some kind of powder -- Unique or 2400 probably -- in a powder measure with a plastic hopper. He left the powder in there for some time, shot up all his .44 Special cartridges, and sat down months later to make more ammo.
At the time, Skelton was a deputy sheriff (he later became a Border Patrol officer).
One day, he got dispatched to where a cow had been hit by a car. To put the animal out of misery, he shot it in the forehead.
The bullet barely left the barrel and blapped off the cow's forehead. The next bullet did too, as did the others.
Skelton was mortified, mostly because he'd been riding around with these bat-f**t loads in his service revolver! Imagine if he'd had to shoot a felon!
His fellow deputies teased him about his, "Cow Killer Loads," thus the name.
He traced the problem back to the plastic hopper on his powder measure. The plastic was frosted where the powder had been lying against it for some months.
Somehow, the plastic reacted with the powder and greatly weakened it.
Since reading that column back in the late 70s or early 80s, I've avoided leaving powder in the plastic hopper of my powder measures -- including my Dillon 550B.
Perhaps today's plastics are different.
Alliant, Winchester and other manufacturers ship their powder in plastic containers, so there are obviously some plastics that don't affect powder.
Myself, I don't like to take the chance.
Besides, if I immediately return the powder from the hopper to its original container, there are no identity problems later.
"Is that hopper full of 748 or 296 ball powder?" I might mutter otherwise, months later.
I think it's just sound reloading safety to empty it after each reloading session.
That's the best reply I've ever seen here. I recall reading that article of Skeeter's too, and I follow those same precautions. Neatness in and around the loading bench is a good measure of safety. Leave it like you found it, meaning put your powder back in the container it came out of. No questions later on that way.