from: http://en.wikipedia.org/wiki/Rounding
Although it is customary to round the number 4.5 up to 5,
in fact 4.5 is no nearer to 5 than it is to 4 (it is 0.5
away from both). When dealing with large sets of
scientific or statistical data, where trends are
important, traditional rounding on average biases the data
upwards slightly. Over a large set of data, or when many
subsequent rounding operations are performed as in digital
signal processing, the round-to-even rule tends to reduce
the total rounding error, with (on average) an equal
portion of numbers rounding up as rounding down. This
generally reduces upwards skewing of the result.