As in life itself - it depends. For US taxpayers that do NOT qualify for a Social Security number an ITIN is mandatory as part of your filing requirements. You must first apply for a SSN and if you get rejected included the SSN rejection letter with IRS Form W7, IRS Form W7-COA, and an income tax return (unless of course you qualify for an exemption to the rule to provide a tax return with the ITIN application W7).