Thanks to all the reply's. For now, I will stick to over until the weather warms up enough to get the garden hose out and test it under. I believe it would be ok, but I really don't want to be wet while trying to sleep, so I'll stick to the better safe than soggy routine.
Doctari, excellent idea. Since I'm using the soft shackle biners, I think if I set the ridgline up without the tarp, I'll snap a couple S-Biners on to make hook up a little faster in case of a sudden downpour.