I wouldn't agitate the hanks--maybe just soak them in Eucalan or Woolite and then spin them out. (And tie them carefully so that there'd be no chance of tangling.) That or I'd hand-wash them.
If there's going to be shrinkage, though, I think I'd rather the garment shrink than the yarn itself; at least the project would be finished. And the way the current one is knitting up, it's a little too big anyway. I just don't like the twiney feel of the hemp as I knit with it. (It does get a little softer as I work with it, though.)
I have heard that both hemp and flax become deliciously soft after a few washings, which was why I dared buy them. Upon first feel, they feel just like a dry loofah.