But efficient planning goes beyond just having some money in all three account types. It also matters where you put different types of investments. Charles Schwab has a good article on tax-efficient investment placement here:
As a general rule, investments that tend to lose less of their return to taxes are good candidates for taxable accounts. Likewise, investments that lose more of their return to taxes may be better suited for tax-advantaged accounts. Here’s where you might consider placing your investments:
Everyone knows not to put munis or annuities in a tax-deferred account, but not everyone thinks of putting REITs in a Roth. REITs aren't taxed at the corporate level, because they pay out a required portion of their earnings as taxable (ordinary income) dividends. But if you get those dividends in a Roth, they're never taxed. The higher ordinary income tax rate on REITs makes a Roth (and, to a lesser extent, a traditional tax-deferred account) even more advantageous for REITs than it is for stocks.
There is one area that Schwab's article misses: foreign stocks (and funds and ETFs of foreign stocks). They should be in your taxable accounts, because you get part of your dividends withheld as tax by the companies' home countries. In a taxable account, you get a credit against your US taxes for the amount withheld. In a Roth, those withheld dividends are lost forever. And even in a traditional tax-deferred account, reclaiming withholdings can't be done until you take distributions and requires decades of record-keeping. It's so cumbersome I suspect few retirees bother.
When choosing your asset allocation, also remember to choose the right accounts!