Should memory cards in DSLRs be formatted every so often?  If so, how often?

I hear that after emptying the card out (transferring images to a computer), that it's a good time to format it to help reduce the chance for error/corruption.  Is this true?


I want to point out:
[Nikon D90 User's Guide by Ken Rockwell][1]

From the site:
> Always format your card after you put
> it in any camera, or if you've
> connected your camera to a computer.
> 
> Formatting your card(s) ensures any
> folder or file corruption acquired
> anywhere goes away.
> 
> You can shoot without doing this,
> however constant formatting is good
> practice and should eliminate ever
> having any card errors. Be sure you've
> downloaded and backed up all the files
> in two different physical locations
> before formatting.


  [1]: http://kenrockwell.com/nikon/d90/users-guide/index.htm