Topshot,
Used osb under mine, and very glad I did. Discussed in this old post: http://talk.newagtalk.com/forums/thread-view.asp?tid=484206&posts=6&highlight=osb&highlightmode=1#M3930321
I liked how it really stiffened the roof up, and I have had NO drips. I would do it again.
Insulation under tool shed roof
