I wouldn't choose one cashier over the other for any condition that is hard to control.
If one of the twin cashiers in the example had makeup that made them look like a freak or something, I'd go for the other, provided the line isn't significantly longer. However, 90% of the time I don't even look at the cashier long enough to form a valid opinion on their looks.
I tip based on service, not by looks. I pay more attention to my phone, the menu, or my food, whenever they come by.