Usually, I use .2 and .25 for the offset and depth.  To my eye, those settings give the most realistic fit.  I have an animation that uses 1 for collision depth on the skirt, because the poor girl's legs kept poking through the skirt hem:

http://www.youtube.com/watch?v=NQDyKbKb2cE

But I can see the gap between the fabric and her hip with disconcerting exasperation!