A girl on Facebook recently commented on my wall asking: Even if a modeling agency were to sign me, would they need my social security number?
For taxes yes, agencies file taxes and they need your social security number to do so, if you wanted to wait until closer to tax time you could, or until the agency asks, but yes you will need to if you made money from the agency from a modeling job. Your agency needs your SS, and your legal name, address.
Remember to save any modeling job payment receipts. You agency might send you a W9 form to use when filing your taxes. And when filing a W9 you are considered an Individual / Sole Proprietor.