Two different issues here:1) If an employer is withholding taxes the way he lawfully MUST do--as well as paying into worker's comp and unemployment--then he needs the social security number of the worker; that's how he makes sure the money is reported property and credited to the correct person.2) If the employer is not withholding taxes, then he doesn't need the social security number--BUT he is committing tax fraud; the employee may be an accomplice in doing so, if he knowingly helps the employer avoid taxes; and even if the employee doesn't face some liability relating to tax fraud, he will not receive credit for social security or unemployment contributions, among other things. (E.g. if fired or laid off, can't collect unemployment insurance.)