In decimal, 1/3 has a non terminating and repeating decimal place of a '3'. In binary, numbers like this exist also.
var myNum = Number(5.1);
At one time there were bugs in the JScript version of toFixed(), but surely by now that is ancient history.
You can play around with some of these formatting issues if you like, using the inputs below. Type in the two numbers and choose the operation from the select drop down and click the button. And a gotcha might be if you format to two decimal places for something like .07 * .01, the result being 0.00.