I end up with better-packed bags at scan-at-the-end self-checkout than with scan-as-you-go or clerk-and-conveyor.

I wander around putting things in the cart, and then at self-checkout I sort and balance it all into bags. If I was scanning as I went, I couldn't carry everything. If I put it on a conveyor belt for staff to scan, then I don't have time to sort or balance when repacking.