Even worse -- Since State and Local income taxes are (partially) deductible on your Federal taxes -- you need to implement those rules no matter if you plan on filing State and Local returns.
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Actually now that I think about it, since the standard deduction is increased, you could catch the majority of simple returns by just assuming the standard deduction and including a few disclaimers about the option to itemize.
The federal deduction for state and local taxes doesn't require knowledge of local rules, just the amounts paid. If you overpay in tax year X and get a refund in tax year Y when filing your local tax return, that's covered in federal tax year Y.
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Actually now that I think about it, since the standard deduction is increased, you could catch the majority of simple returns by just assuming the standard deduction and including a few disclaimers about the option to itemize.