In my own shopping, I just use whatever seems likely to be fastest or less hassle -- for me.
If I've got a bunch of stuff, I'm heading to the cashier because they're better-equipped to handle a volume of stuff than I am at self-checkout.
But if it's just a couple of small items, then the self-checkout seems fastest: Scan, plonk, scan, plonk, invoke the incantation so that it can take my money[1], and pay it.
(Unless one of those small items involves something like beer or something else requiring an ID check, wherein: It's back to the cashier.)
[1]: In my neck of the woods, Wal-Mart gets this best, with as few as one button-pushes required to pay and leave (and there was a time when it was zero button pushes to use a debit card at self-checkout there). Dollar General gets it worst, requiring at least 8 button pushes (with three different input methods! it requires input on two different touchscreens and one physical keypad) to pay them and get on my way.
It is?? I missed that memo. But in my part of the US, the manned cashiers always have a line for them, too.
its empty because it’s considered rude to burden the poorly paid checkout person when you can self-service. Once all of the self-service lanes are occupied, the sentiment shifts and customers uneasily queue up in the manned lane, frantically watching if the self-service lane clears up so they can jump out of the manned lane.
One of those freak cases in behavioral econ where the causality is very straightforward and explicit.