Many people may know how to make calculations in mind, but using a calculator would be best. The exception of use is elementary calculations, such as when the person owes $2 and pays a $5 bill. Most likely, the cash register has a built-in calculator, which makes the job easier, but if there is no calculator, opt for the progressive counting method. For example, you received a $20 bill for something worth $14.50, so you start with two quarters to round to $15 and then a $5 bill to get to the $20 initial.
Here is an excellent video that demonstrates how to work when you have a built-in calculator in your cash register:
Source: Work - How to Give Back Correct Change