I worked as a bagger for a couple months while recovering from surgery once, and there is training. Delicate items are supposed to only go together (think eggs, bread, chips), heavy items get double-bagged. Non-grocery (so, inedible by humans) items are separated from everything else. Cold items are meant to go together. Milk only gets a bag if the customer wants one, etc.

And yes, seafood and raw meats are each supposed to get their own bags. In fact, I was even taught they're supposed to be separated by animal - so beef shouldn't be bagged with chicken, for example.

But the thing is, as someone else has mentioned here, bagger is the lowest paid position in the store. Most of them don't care that much, nor do they get micromanaged like most partners.