1/5 in decimal is 0.00110011... repeating in binary, therefore not exactly representable, therefore it could lead to errors. Not big errors, but enough that smart people would prefer not to use it for money.
To give a concrete example, 19.99 can't be exactly represented in floating-point, and ends up being 19.989something. If you have a price of $19.99 that's represented in floating-point, and if you truncate rather than round as you pass it off for display, you'll end up displaying a price of $19.98. With enough manipulation, the error can accumulate to the extent that rounding the result won't save you either.
