It depends on your gear, pretty much. From what I've been seeing, once you get the legendary meta you'll be going full haste pretty much. Until then you have to balance haste and mastery.
More like going all out mastery.
Until you got an average itemlevel of 522+ and are wearing 2 of the trinkets that drop in ToT Master>Haste=Crit stays true. Usually even Expertise takes over Haste and Crit, so go for the 7,5 % softcap.