Originally Posted by Sheldon Brown
The stem shouldn't be overtightened. It is normal that if you hold the wheel between your legs and yank hard the stem should move.
That doesn't mean it will actually move while you're riding, since there's normally very little resistance to rotating the fork in actual riding.
Some folks do overtighten the stem, but there are a couple of bad consequences to this:
•If the bike falls over or crashes, there's a greater chance of bending the handlebar or wrecking the brake lever if the stem is totally immoble.
•With a quill stem, there is a risk of deforming the fork steerer, causing it to bulge, if you overtighten the stem.
Sheldon "Don't Get Carried Away" Brown
Brings up something I was musing about eariler. Threadless clamps hold the steerer pretty damn firmly, which I always considered a good thing. (Never have to readjust) But then... it stands to reason that the bar can get damaged easier. Should I leave the bolts loose enough to be able to turn the bars?