I used to worry about when to mark returns as "final" but I now think that all the "final" box does is tell the IRS not to poke you about filing or perhaps not send you 1041-ES if you were once making estimated trust payments. If you get a 1099 in a year after a "final" return, I can't image the IRS won't come after you. Even if you check final, I have read of people filing another return (after the year marked "final") in later years when a 1099 or other time of income unexpectedly appears. They report no problems doing so.
The only wrinkle does not apply to a 100% grantor trust. Rather for a non-grantor trust certain deductions are passed out to the beneficiaries only in the final year. So for a non-grantor trust with such deductions (not all trusts have them) it might be problematic to have to file again in a subsequent year. I.e. would the trust (and beneficiaries) need to amend the final year and move those deductions forward? That would be a mess.
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