Here is what Wikipedia has to say on the subject:

"""
Round half to even .. This method treats positive and negative values symmetrically, and is therefore free of sign bias. More importantly, for reasonable distributions of y values, the expected (average) value of the rounded numbers is the same as that of the original numbers. However, this rule will introduce a towards-zero bias when y − 0.5 is even, and a towards-infinity bias for when it is odd.
"""
https://en.wikipedia.org/wiki/Rounding#Round_half_to_even