Works for most light bags, many people use heavy reusable bags which can sometimes throw off weight of some items too much and it's a lottery then. Similarly I don't use "add X items at once" because they don't multiply weight error by X so for many items it sometimes do not work.

I just pile my junk on the scale and bag it up after I'm done with the transaction.

This does mean you block the person behind you for much longer, but that's the store's problem not mine.

If it means there's never an "unexpected item in bagging area" it probably overall reduces time.

I do this too, but most stores have plastic bag holders on the scale that clutter things put and make it harder to stack items before bagging.

I imagine its regional, but where I'm at those are long gone. Now you gotta bring your own bags or pay for each of the ultra flimsy 'reusable' store brand bags.