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 wanted to point out: Nikon D90 User's Guide by Ken Rockwell

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.