A shocking number of people (edit: who implement billing related software) are unaware of how many decimal points of accuracy their local tax code requires to calculate vat or sales tax correctly. And those things are often specified in terms of arithmetic correctness.
I had our CFO stand behind me while I talked him through every step of our VAT calculations once, because he was legally responsible if I made us round it wrong, to the wrong number of digits. And had we e.g. done something grossly incompetent like used floats for those calculations it most certainly would have been wrong, but so would it if I used fewer than five digits past the decimal point or failed to round in the right direction after that.
It's usually not hard, but it requires being aware that you need to look up the right rules. And know better than using floats.
I had our CFO stand behind me while I talked him through every step of our VAT calculations once, because he was legally responsible if I made us round it wrong, to the wrong number of digits. And had we e.g. done something grossly incompetent like used floats for those calculations it most certainly would have been wrong, but so would it if I used fewer than five digits past the decimal point or failed to round in the right direction after that.
It's usually not hard, but it requires being aware that you need to look up the right rules. And know better than using floats.