In WyCash we eventually refactored all imprecise and rounded arithmetic out. What a relief. wiki 
If we saw a disagreement in the 10th decimal place someone would say, "It must be rounding." I would say, "It can't be rounding, we don't round anything." Then we would get on with tracking down the problem.
Our implementation used unlimited precision integers for most numbers, but sometimes stored prices or ratios as rational numbers (an unlimited numerator over an unlimited denominator).
The cost was unmeasureable in speed or space, a fact that surprised me. It seems even computationally intense business applications simply don't do enough arithmetic to justify anything short of exact.