As a former employee of Citi cardmember services, I can tell you that we would hope that you sign the back when instructed to do so (when you call to activate the card) so that should there be a fraud claim, it will help in the investigation.
Many people do not sign the card because they feel that should their signature be mastered by a thief, it will make a claim harder to prove. Stores arent supposed to accept an unsigned card, but most do, as you have said, because they have a green interest.
If you were truly that concerned, maybe a card with a photo will be better for you.
If it helps at all, just know that even though many people claim fraud, many are not fraud, just people not wanting to pay and report it stolen or lost, etc. Claims are not signed off easily, like money off a tree. They are investigated fully, and if someone on the video surveillance isnt you, I wouldnt worry about the signature, present or not. If every credit card in your history has claims on it, thats a problem. If not, thats what the buyer protection is for.