I think the one big advantage to a cashier wearing a mask is as a reminder not to touch their face. Unfortunately, a cashier is probably at highest risk to get virus particles on their hands, as they are constantly touching products (to ring them up) and cash, which has been touched/handled by many others.
Even worse is the person at Costco that checks your receipt on the way out. It is one or two people at the door, who now have a contact point with every single individual who purchased something in the store. If a person had virus particles on their hands, and touches the receipt, then transfers it to the glove of the receipt checker, that glove is now potentially transferring virus particles to receipts handled afterwords. I have started just holding up my receipt for them to look at (through the plexi-glass) rather than passing it under the slot for them to handle.