As a recently promoted manager of a big retail store, yes.
We teach our employees that if a customers card is NOT signed and it prompts them up to enter the last four digits (usually an amount over $25), they are to check the customers ID. If the card is signed and we ask for ID, we can technically be fined for breaching merchant agreement, but that's a whole other story that many of your average Joe's don't know about.
From having held about 3,000 credit cards in the past 2.5 years, the safest way to present your card is to have "SEE ID" written on the back. If your card is signed, anyone can go to town with it and not have to present their ID. We see it all the time. Had this one guy come in and purchased about 5 Macbook Pros with a clearly stolen credit card and we had to accept the transaction.... Only because there was a signature on the back of the card..... Of course after the guy left the store we called in a special code and the card came back as fradulent and they caught the guy. 
But we are only one out of a very few other retailers that actually do this practice.
@Anonymous wrote:As a recently promoted manager of a big retail store, yes.
We teach our employees that if a customers card is NOT signed and it prompts them up to enter the last four digits (usually an amount over $25), they are to check the customers ID. If the card is signed and we ask for ID, we can technically be fined for breaching merchant agreement, but that's a whole other story that many of your average Joe's don't know about.
From having held about 3,000 credit cards in the past 2.5 years, the safest way to present your card is to have "SEE ID" written on the back. If your card is signed, anyone can go to town with it and not have to present their ID. We see it all the time. Had this one guy come in and purchased about 5 Macbook Pros with a clearly stolen credit card and we had to accept the transaction.... Only because there was a signature on the back of the card..... Of course after the guy left the store we called in a special code and the card came back as fradulent and they caught the guy.
If a credit card says, "Signature Required - Not Valid Without Signature" - and many of them do, writing 'See ID" or "CID" on the back won't work. My local USPS is particulary 'funny' about this, and I've seen them refuse to take a card more than once (they make you hand it to them after swiping it yourself).
There are numerous threads on this with various opinions, and here's mine: I want to make my purchase with as little "drama" as possible, so my cards are signed.
I have found that arguing the policy only gets the clerk/cashier more 'entrenched', and it's not my goal (or job) to educate various cashiers on the finer points of MC/V/D/Amex merchant contracts.
If there's fraud, that's the credit card issuer's problem - not mine. The credit card issuers are paid BIG bucks to deal with that kind of thing, and since we have the abiliby to view our accounts in real-time it's unlikely a fraudster could get very far, anyway.