Disclaimer: You will need to sim your gear to get 100% accurate results:
http://code.google.com/p/simulationcraft/downloads/listThe general trend I have seen is that (espec in simcraft) is you don't want to go under 2,000 haste, mastery is your #1 secondary with 4p t13.
t13 4p: Int>Hit/Spirit to cap>Mastery>Haste>>Crit
Prior: Int>Hit/Spirit to cap>Haste>Mastery>>Crit
If you go under 2k haste a good general rule of thumb is to reforge back some of the mastery back into haste, if you are under it is a slight dps decrease that quickly descends.