You guys are dorks. For one, IO clumps up, so rolling it makes it easier to get it out.
Usually, the chemicals don't separate out, but
sometimes it is an issue. I test every single bucket top and bottom for the last 10 years or so. I've had a handful of buckets of IO with high Mg at the top and low Mg on the bottom - it's really happened! Furreals!
IO and IORC are the only ones I roll. H2Ocean and Tropic Marin are packed tightly in bags, and I've never tested a bucket with settling issues. I only test the "big three" though.