Could you post a specific example of when a rounding issue occurs?
My math might be a bit fuzzy, but let's say tax rate is 15% and X is a monthly payment before tax. Whether you do X * 12 * 0.15 or X * 0.15 * 12 - it's the same number. So the only issue can be either a tax rate change or instead of starting from X and building the total amount by multiplication you come up with some total amount and then divide it by 12. That's what we in the biz call "shooting yourself in the foot". 