Many years back I worked on a project involved in heavily government regulated gambling (horse race betting). The rules were every calculation including all intermediate steps had to be done to at least 1/10,000th of a cent precision, and rounding was only allowed once at the very final step.
(Which, since my part of that project was a Javascript/jQueryMobile mobile webapp, I managed to push all that to the backend team saying "I'm doing calculations using Javascript, I don't even have proper integers to work with here - you'll need to send me all financial figures as strings, and do all the calculations at your end according to the rules. I can accurately display strings, I will not look a judge in the eye and try to claim I can accurately calculate _anything_ monetary in Javascript.")
(Which, since my part of that project was a Javascript/jQueryMobile mobile webapp, I managed to push all that to the backend team saying "I'm doing calculations using Javascript, I don't even have proper integers to work with here - you'll need to send me all financial figures as strings, and do all the calculations at your end according to the rules. I can accurately display strings, I will not look a judge in the eye and try to claim I can accurately calculate _anything_ monetary in Javascript.")