Be careful using traditional 401(k) just to avoid NIIT instead of contributing to Roth 401(k). If the 401(k) performs well, you might find your balance is so large that even with reasonable Roth conversions you're setting yourself up for IRMAA and higher tax brackets (and NIIT!) when you take RMDs. (Nice problem to have.) You might get the traditional-Roth tradeoff right if you have a crystal ball that shows you future tax law and your future cash needs, and your doctoral thesis was in linear programming.
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Be careful using traditional 401(k) just to avoid NIIT instead of contributing to Roth 401(k). If the 401(k) performs well, you might find your balance is so large that even with reasonable Roth conversions you're setting yourself up for IRMAA and higher tax brackets (and NIIT!) when you take RMDs. (Nice problem to have.) You might get the traditional-Roth tradeoff right if you have a crystal ball that shows you future tax law and your future cash needs, and your doctoral thesis was in linear programming.
Post: Investments Tax
Link to comment from January 31, 2026