It seems to me what makes sense is to try and estimate how much taxable income one will have - distributions plus SS - and try to even out that income stream over time. Because the tax structure is progressive, the lowest total tax over time will be paid when the income is always taxed at the same rate. One extra dollar of income is taxed at a higher rate than one deferred dollar, so "tax volatility" leads to higher total taxes paid.
Ideally, assume 30 years span, SS begins in 10 years, calculate the real PV of both those income streams, then calculate the tIRA withdrawal flow so that each year is the same inflation adjusted amount over the entire 30 year period. If that leads to a withdrawal of unneeded tIRA, then convert the difference. Any shortfall between budget and withdrawal is made up from the taxable funds. The Roth are the last to go because they are entirely tax free.
Ideally, assume 30 years span, SS begins in 10 years, calculate the real PV of both those income streams, then calculate the tIRA withdrawal flow so that each year is the same inflation adjusted amount over the entire 30 year period. If that leads to a withdrawal of unneeded tIRA, then convert the difference. Any shortfall between budget and withdrawal is made up from the taxable funds. The Roth are the last to go because they are entirely tax free.