Originally Posted by mitchc
Be wary about using grease on critical bolts(mains, big ends, head bolts) you can change the clamp load characteristic of the bolt which can cause snapped/sheared bolts when taken up to the recommended torque.
Lots of opinions on this one. One school is that the torque holding force is due to the threads tightening down on each other; another is determined by a combination of friction in the threads and the head of the bolt, and a certain amount of stretch; another is that it's all in the stretch (which distorts the threads, locking them in place)More than likely there are a bunch of others.
Use of grease or advisement against doing so would depend on which school you believe.