Matt, I disagree.
By the time that you reach an age where you are counting change back, you've experienced change being counted back countless hundreds of times. If you've not paid attention, it's not your boss' fault. If you've never seen change counted back, your parents aren't doing their job.
As to staring at a register, when I worked at a service station, I observed people. People liked it when they walked in, and their cancer stick of choice was on the counter before they got there, or I told them where the coldest drinks were. Also made sure I'd added up their total in my head before I started keying in the register.
Little tricks I developed to avoid getting bored, and sharpen my mind for university.
One of my scariest experiences was buying a loaf of bread and a litre of milk. The girl typed them into the register, and told me that it was $298 (two hundred and ninety eight dollars).
I said that she was wrong, and she turned the register to me to show me that it said $298.
That's not the manager's fault that she couldn't fundamentally realise that something around $2 and something around $1 could never tally something around $300.
When I'm tutoring, as soon as they pull out the calculator, I make them put it away.