To maximize the benefit of Roth conversions, one needs to have a fairly accurate estimate of AGI and taxable income before the end of the year, so it's unreasonable to suggest that doing that calculation before the end of the year is a waste of time. For example, someone subject to IRMAA on Medicare Parts B and D could incur a $1000 Medicare-cost hit by going over the AGI threshold for their current IRMAA bracket by even $1; going over by $1 would be equivalent to that $1 being subject to a 100,000% (yikes!) marginal tax rate. Other important thresholds include the top of the current income tax bracket, the AGI threshold where NIIT kicks in, the taxable-income threshold where LTGCs get pushed from the 0% CG tax bracket to the 15% CG tax bracket, etc.
