We also arrange things on the conveyor according to a preferred system. Gallon milk jugs, large juice containers, and other heavy items like a sack o' 'taters, bag of apples, large peanut-butter, case of soda-pop/bottled water, etc - those all get placed in the cart in a way that they are easily accessible first when it's time to load the conveyor. Rule #1: No heavy stuff crushing other items when the conveyor crams things together.
Canned items are next onto the belt. Then boxed and bagged items, keeping frozen and refrigerated items together. Unpackaged produce, chips, eggs, and bread go on last.
We prefer to bag ourselves, as the baggers inevitably do it illogically. When we DO end up at a store with a bagger, pet peave: baggers leave as many things out of sacks as possible. Milk jugs can easily be packed two-to-a-bag, and the bag WON'T break unless you mishandle it. But they put the jugs in the cart solo and I have to take them out and bag them myself. 6-packs of Gatorade, or any container with a handle - they don't bag those either. I'm interested in as few trips from the Excursion as possible when unloading at home. 4 Gatorade 6-packs, 2 OJs, and 4 gallons of milk, can either be 10 separate items to wrestle with, or 5 bags. I prefer 5 bags. Bagger-boy doesn't see it that way.
We use a mixture of plastic and paper sacks, as both come in handy at home.