I worked in a grocery store in a small town when I was in high school (which means we bagged and carried out for everyone unless they specifically said they would get them). It sounds easy but you'd be amazed at how badly most people are at it (including the managers who would criticize us about it). There were a few people who would go out of their way to specifically say how to sack something (keep bread separate for instance) because some of the workers were too stupid to not do it (like fill a paper sack to the top with canned/bottled stuff which guarantees that it will rip). It really wasn't a bad job other than the pay (I didn't have a problem with customers), which was better than minimum wage so I can't complain too much.
We get our groceries from Wal-Mart now so it depends on how much they do. Most of the cashiers are decent and will do an ok job, but some are horrible. We had a really bad one recently and so if we see her anymore we get in a different line even if its longer (which we still usually end up getting through faster because she's so terrible at her job).
I actually like sacking my own groceries now as I don't mind the little extra work and it makes it easier for the cashier anyways.