Basically, you just want to keep them from drying out. Better dryer than wetter because at this point, they have no way of getting rid of any excess. Once they are up and growing and have a bunch of leaves, the leaves provide a method for the plant to lose moisture. At this same time, the rhizome should have produced it's own roots that allow the plant to obtain more moisture from deeper in the soil. I never end up doing much if any watering and can't remember any of them drying up and withering away. Of course, we generally get an inch or so of rainfall on a weekly basis. Just don't water too much or they'll drown.