As someone who has been the victim of card fraud on no less than 7 separate occasions, I can tell you, it makes zero difference whether you sign the card or not.
Firstly, nobody checks the signature. The vast majority of cashiers either don't have the time or just can't be bothered. I say that as a part-time cashier myself. I'm not a police officer and I'm not paid enough to argue with a potential thief simply because someone's handwriting looks a bit off.
Secondly, copying a signature is a ludicrously easy thing to do. In this day and age, we really need to get away from the idea that you can positively identify a person by the way they write their name.
In my experience, the vast majority of fraud these days takes place without the thief taking possession of the physical card.
The card is either cloned by a skimming device attached to an ATM or POS machine, or increasingly, wirelessly read by portable scanning devices.
Then the details are imparted onto a duplicate card, which the thief can then use with impunity, until it gets blocked (or maxed out) and transactions start getting declined.
Otherwise, the card number is simply used online. The most low-tech fraud perpetrated against me was about 15 years ago, when a contractor at my dad's house copied down my card number and used it to run up a couple of grand on phone-sex lines.
The bottom line is, there is simply no way to protect yourself 100% against card fraud. I have just given up now and accepted that, every now and then, I have to go through the rigmarole of reporting fraudulent transactions, signing declarations to get the money back, and ordering new cards.
Frankly, it's less hassle to have to do that every few years, than it is to try to use every protective measure in the book on an everyday basis, which quite honestly is a losing battle.