SecondCor521
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This is true, but due to the way our taxes work, this nearly always works against Roth's since the contribution/conversion is always at the highest marginal rate in a given year while incomes in retirement always are starting at the lowest marginal rate. The overwhelming majority of folks are better off ignoring Roth's for that reason and the fact that that your highest marginal tax rates in retirement for the overwhelming majority of [A]mericans is lower than your peak years.
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Many people here understand taxes pretty well.
I think your statements above are misleading.
Yes, income in any given year always fills up brackets from the lower to the upper. However, when making a decision whether to Roth convert on the margin, those additional Roth conversion dollars are either added at the top in the current year or added at the top in a future year. So it's marginal rate now versus marginal rate later.
And many posters here, including me, will have our highest marginal rates in retirement due to SS, RMDs, investment income, inheritances, and any other income sources. Having enough to retire early, living on 4% or less, having average investment luck over a few decades, and not doing Roth conversions can create very high marginal rates.
Finally, the comparison for Roth conversions doesn't have to be peak earning years versus RMD years. It's early retirement years versus RMD years. So personally I made a lot of tax deductible 401(k) contributions during my peak years and am doing Roth conversions in my early retirement years. These conversions are thus done at a 0% federal income tax cost compared to 24%+ during either peak or RMD years.