From your comments,
the tax amount rounded to the 4th decimal and the total price rounded to the 2nd decimal.
Using the example in the comments, I might foresee a case where you have 400 sales of $1.47. Sales-before-tax would be $588.00, and sales-after-tax would sum to $636.51 (accounting for $48.51 in taxes). However, the sales tax of $0.121275 * 400 would be $48.52.
This was one way, albeit contrived, to force a penny's difference.
I would note that there are payroll tax forms from the IRS where they do not care if an error is below a certain amount (if memory serves, $0.50).
Your big question is: does anybody care if certain reports are off by a penny? If the your specs say: yes, be accurate to the penny, then you should go through the effort to convert to DECIMAL.
I have worked at a bank where a one-penny error was reported as a software defect. I tried (in vain) to cite the software specifications, which did not require this degree of precision for this application. (It was performing many chained multiplications.) I also pointed to the user acceptance test. (The software was verified and accepted.)
Alas, sometimes you just have to make the conversion. But I would encourage you to A) make sure that it's important to someone and then B) write tests to show that your reports are accurate to the degree specified.