A new, dry, unlubricated bolt will present more resistance to the torque wrench due to the increased friction of the threads and bolt head upon contact with the compressed part. It will also present an incorrect measurement of the tension on the fastener, which is what you're trying to accomplish. On the other hand, a used bolt, who's contact surfaces have been polished by previous tightening will present less resistance.
The short answer is I always clean and lightly lube bolts I am tightening, except when using loctite or anti-seize since these materials exhibit lubricating properties on their own.
'98 KDX 200
St. Louis, MO
[This message has been edited by dhoward (edited 04-17-2001).]