Second, filling the 12% bracket with Roth Conversions can be a great idea, with one caveat. There is a phase-in of taxes on long term capital gains (LTCGs) that starts when your AGI (sum of ordinary income + LTCG) hits the top of the 12% bracket. If you do Roth Conversions beyond that, then those last dollars are themselves taxed at 12%, plus they push an equal amount over the capital gains phase-in, so you pay 15% on that. Meaning that those last $s are taxed at 27%!
Once all of your LTCGs are taxed, then your marginal rate falls back to 22%. So folks doing Roth Conversions always have to decide whether to stop just before LTCGs get taxed or push through that 27% zone to get to the 22% bracket.