While the first safe withdrawal rate research originated nearly 20 years ago, it was simply based on an analysis of the sustainable cash flows that could be drawn from a portfolio over a 30-year time horizon based on various historical scenarios, ignoring the impact of fees and taxes. Of course, in the real world clients must bear the cost and impact of both, which reduce not only the amount of growth that the portfolio enjoys over time, but also the level of sustainable cash flows.
In reality, though, extensive research has been done over the past two decades to model the impact of taxes (and fees) on safe withdrawal rates. The impact is not as severe as some might fear - it's not as though the safe withdrawal rate must be chopped by 25% for a client in the 25% tax bracket - due to the fact that taxes have a natural self-mitigating effect, as they are only due when the portfolio has appreciated (which is when it has the most money available to pay those taxes). Of course, if the account actually is a pre-tax IRA, though, the net spending really does have to be reduced for the entire marginal tax rate!
Although the application of these adjustments can become complex when combining taxable accounts, pre-tax retirement accounts, and tax-free Roth IRAs, the fact remains that a proper application of the safe withdrawal rate research should reduce a client's recommended spending to account for the future drag of taxes, along with investment expenses and advisory fees. On the other hand, the good news is that other factors can help to boost the safe withdrawal rate as well, including a client's greater diversification (i.e., more diversified than the original research assumptions), spending flexibility, and varying time horizon. Ultimately, all of these adjustments can be incorporated to arrive at an appropriate safe withdrawal rate recommendation for a client.