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@Sam55 wrote:

In public accounting tax work, it is common practice to leave boxes 16a and b blank

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I don't know what your common practices are, but you aren't supposed to pick-and-choose which parts of a tax return to fill out.  Do you really sign a tax return that you know is not "complete"?

Just like many other things on a tax return, you ask the client the question and record their answer on the tax return.  If you don't believe the taxpayer, are you sure you want to be preparing their tax return?