90% of the fastening strength bolts/nuts/washers comes from the hex head against the nut. This is why you use a washer. Lubricating a fastener should only been done if spec'd out or if there is a corrosion or galling issue.
For some more info do a search on the attached site:
http://www.eng-tips.com/viewthread.cfm?SQID=57211&SPID=68&newpid=68&page=1