When applying for an American Express card, it asks me for my social insurance number (SIN) but it is optional. Should I give it? The site states
We encourage providing your SIN for more accurate identification (e.g. obtaining your credit bureau report).
As an aside, I remember when I got my SIN card I was told it is confidential and to keep it a secret. I can think of lots of things that ask for your SIN quickly, for example some job openings ask for it along with the pre-screening questions. What bad things can someone do with your SIN?