I am neither a resident nor a citizen of the United States, but about twenty years ago, I did live and work there temporarily. I got a Social Security Number and paid income tax on my earnings from that job. Nowadays, as a non-resident, I occasionally do freelance writing jobs for American publishers. Each job comes with a fairly simple publication contract that says the publisher agrees to pay me $x in exchange for first publication rights to my work. The contracts always leave room for my signature, the date, and my SSN.
As a non-resident freelancer performing my work entirely outside of the US, am I actually required to provide my SSN on such a contract? As far as I know, my only tax liability on this income is in my country of residence, not in the US. Regardless whether or not I am required to provide my SSN, are there any benefits to doing so? If I am not required to provide my SSN but I choose to do so anyway, are there any disadvantages?