Certainly in December you'll have a better idea of where you are in your current tax bracket and be able to adjust your Roth conversions to try to top that out. Going over a bit usually isn't too terrible since only the portion of the income that is in the next tax bracket gets taxed at the higher rate. Still, because of various potential side effects of an increase in AGI you'll want to experiment to find the actual marginal tax rate on each increment of income. You might find that the marginal tax rate goes up for one increment and goes back down some for the next increment. People who receive Social Security income and have not reached the income lever where the maximum 85% of Social Security income is taxed experience this, but there are other ways this can happen. Don't wait until the last minute in the year to do a Roth conversion, though, especially if your custodian is not one who generally processes transactions like this the same day or the next day.
You could do several conversions during the year, doing one final conversion late in the year to get close to your target.
TurboTax is usually working well enough by mid-December to get a pretty good estimate, but the CD/Download version provide much more flexibility to experiment than does the online version. The CD/download version also allows overriding values that have not had their calculations updated for the new year. For example, despite the amounts having been known for well over a year, the developers always seem unable to get the HSA contribution limits updated until mid- to late-December, but overrides can deal with that.
