At the risk of repeating an opinion I've offered in other circumstances: With these withdrawal strategies, I don't think the objective for most people should be "minimizing taxes" or even "maximizing withdrawals". For me, at least, the goal of the whole exercise is to minimize the risk of running out/low on money on later years. Now, when we start down the road of trying to fine-tune 401K or tIRA withdrawals to minimize future taxes, we have tp make a lot of assumptions about future tax rates, future tax brackets, and growth of investments. Even a tiny change can make a big difference in the optimum withdrawal strategy. And, honestly, none of us can accurately guess about tax policy or investment returns over the next several decades. So, trying to fine-tune this whole thing beyond a certain very basic degree is a fool's errand.
More importantly--our nest egg has to see us through decades of uncertainty. If I avoid paying taxes now, that's more money to serve as a cushion against that uncertainty. Maybe my investments will do terribly and we will have low withdrawals as a result: if still have a progressive tax system, then I won't pay much in taxes. On the other hand, if my investments do great, then when we are really old (and future financial uncertainty is reduced--'cause we're nearly dead), then I don't care much about paying a higher tax rate. The money served its purpose, we made it.
So, when trying to figure out how much to take out in tIRAs vs Roth IRAs, I bias toward paying the taxes in later years once in the grey area of the calculations.
Another thing to consider when running "what-ifs"--a surviving spouse. The std deduction and tax brackets are a lot lower when file single, and the tax bite is a lot bigger. If the finances will be a due to the taxes, this is an argument for keeping more money in the Roth and depleting the tIRA.
