Found this here:
http://www.straightdope.com/mailbag/mcredit.html
Dear Straight Dope:
The other day I read a report explaining why we should not sign the back of our credit cards so we will be forced to show our drivers license when we use it. That's suppose to protect us just in case someone steels it. But, the back of the credit card indicates that the card is not valid unless is signed. Who should we believe? --Derek0701
SDSTAFF Ken replies:
You should believe me, as I have worked in retail, for a company that issues its own credit cards. Here's our official advice on signing the strip on the back: Don't sign it. It's useless as a deterrent, as anyone who takes your card then has a sample of your signature which they can not only use on any charge slip, but on your checks as well. However, do not leave the white strip blank. In that space, write: "Ask For Picture ID," and be prepared to back that up someday when you're in a hurry and the clerk wants to see a driver's license as well as the card. It makes the charge transaction a little longer, but a lot safer.
SDSTAFF Dex adds:
I concur with Ken. That's what I was told when I lost a card, which I didn't realize until I got a statement with purchase of furs, $3000 or so. Their signature was a reasonable facsimile of mine, I mean, I could tell it wasn't mine by looking, but a clerk looking quickly wouldnta. (Turned out OK, I had to sign a notarized affadavit that I didn't buy them ... and since I was out of the country on the purchase date, I
had proof if needed). In the aftermath, the store suggested exactly what Ken said, writing "ASK FOR PHOTO ID" in the signature block. Occasionally takes a little more time, but avoids later hassles.
--SDSTAFF Ken and SDSTAFF Dex
Straight Dope Science Advisory Board
What do you think is the way to go Sign or No ??