It isn't. Ever. If you must mash something, mash 5A (though I would not recommend it)
Few notes:
One of the last things you want to do against hakumen is rush him when you have a meaty 214D, because after a neutral tech, he can just 6D the 214D and combo you if you're in range. Also, hakumen cannot combo after 6B at all (he can't even RC the move) unless you don't tech, however, if he does land it, he's at considerable advantage.
As for getting out of melee range, pretty much treat it like anybody else. Block, tech throws, look for your opportunities out. Don't expect to mash out of anything, because most of hakumen's normals are + on block, and there are usually few holes in his stings. Thankfully though, his strings are pretty short. Quick note: you can DP between hits of renka (low spinkick thing) if you instant block the first hit. Gurren (dash punch) can be punished with 5a if you instant block it.
As far as what not to do when you're pressuring him at melee range, just don't 2C > 6C, or 5C > 6C. You'll just get countered, and that will make you sad.