There's some research that indicates that the "venting anger" theory leads to the opposite, i.e. more anger, not less.
[1] http://www-personal.umich.edu/~bbushman/PSPB02.pdf (quick google, there are certainly better and more up-to-date papers).
[1] http://www-personal.umich.edu/~bbushman/PSPB02.pdf (quick google, there are certainly better and more up-to-date papers).