..writing instructions is silly, as once you are in a foreign country and the clerk, who may not speak English, tries to match your signature with the bill.
..if the bank has your signature for the card on file, it might come in handy if there is a dispute whether or not fraud was involved. however, in this case it is smart that the card wasn't signed.
..as above commented, credit card institutions usually do not verify signature against the bill copy they receive, unless some alarms were tripped. they also detect fraudulent behaviour using algorithms to match against your usual shopping behaviour, geographic locations, and known scammed shopping patterns.
I usually, don't sign my cards until the moment a clerk somewhere in the world requires it and I can't talk him out of it with my IDs. I unintelligible scribble the bills as well, if I know they won't check against the card and there is a risk of a dodgy operations, as some countries warrant. then, I can easily deny all the charges.
(further: make sure your credit institution has your signature on file)
(more further: don't use liability unprotected cards in locations you don't trust; better don't get the card in the first place if it isn't)