Coinfloor uses an approach called "weighted stochastic rounding" to calculate the fees charged to our users when trading.
Weighted stochastic rounding over time minimises the effects of rounding errors.
To understand why this rounding method is preferable to traditional "round-to-nearest" methods, consider the case of one hundred occurrences of a 4.71 fee, each rounded according to the nearest whole number. Using the weighted stochastic rounding method, the expected value of the sum of these rounded figures is 471, which is exactly equal to the ideal sum of the unrounded figures. Using the traditional method, the expected value of the sum of the rounded figures would be 500, which is far from ideal as it results in a much higher fee charged.