My earlier reply had to do with making sure one avoids an underpayment penalty if one does a Roth conversion early in the year.
As fanfare suggests, there is more to deciding when to do a Roth conversion than determining when you will need to make an estimated tax payment. As fanfare suggests, if your IRAs are invested in stocks or stock funds, converting when the market is down and is subsequently expected to rise will allow more shares to be converted for the same tax liability than a conversion done after the rise, resulting in an overall tax savings since the subsequent rise will be tax-free when the requirements for qualified Roth IRA distributions are met. Of course that means that you are trying to time the market to some extent. Doing several Roth conversions throughout the year would be the equivalent of dollar-cost averaging.
