I heard porn surfing increases workplace productivity. Or harassment. One of the two.

The problem with white lists is that you might be getting phone calls a lot for who-knows-what site that people heard about and they think it's relevant. Sure, adding it to the list would be a matter of seconds - after, of course, checking its content - but it depends how much time you have for that.

On the other hand, somebody might be setting up a tripod page with some tools/exploits to rig their local machine [from home] and once they get to work they start hacking [in the sledge-hammer sense] at their box with said tools. And blacklisting all of tripod doesn't strike me as a great idea [or geocities, for that matter].