Optimal sequences of portfolio withdrawals

[Nords]

FPA's published a study on "Extending Retirement Payouts by Optimizing the Sequence of Withdrawals" with these conclusions:
1. When harvesting money from two assets with the same tax treatment but whose annual rates of return are different, the annual withdrawal should be taken from the lower-performing asset first.
2. When withdrawals are to be taken from a tax-exempt and a tax-deferred account, the sequence of withdrawals is immaterial. That is, the shortfall risk remains unaffected by the order of withdrawals as long as both accounts grow tax-deferred at the same rate of return.
3. When withdrawals are to be taken from a tax-deferred account and a taxable account, the optimal withdrawal strategy depends on the expected rates of return of the individual assets in these accounts. Withdrawals should be made first from the account with the lower expected (post-tax) rate of return. When making withdrawals, it is not true that the tax-deferred account should always be left for last.
4. Owners of Roth portfolios can potentially extend the withdrawal process by simultaneously withdrawing from the Roth and from a tax-deferred account.
5. Perhaps the lesson to be learned is that the "obvious" answer is often not the correct one. The advantages that are presumed of tax-deferred and tax-exempt accounts during the accumulation phase do not necessarily carry over into the withdrawal phase.