Heres another thread on jam nuts.
http://www.streetsourcemag.com/forum/topic.aspx?topic_id=95934
I'll quote Max Fish on this topic:
"A thread that isn't tight is exponentially weaker then a properly tightened part."
I guess what I meant to say is that I would rather slightly overtighten a nut or bolt than just leave it loose by not tightening it at all.