Just my personal experience in my house. I have a bath vent that vents out the eave of the house and I get mildew issues on the underside of the roof decking there where ther exhaust exits. I am planning on changing this soon to a roof vent.
Thanks for chiming in. That seems to be the consensus I'm reading on the net. In my case, the soffit is not flat/perpendicular to the wall. It follows the roof rafters back, so there's a "triangle" of space there that will probably collect moisture. That being said, I would still like to work out a way to do it. Much prefered to running it directly out a side wall.