
The goal is to maximize overall after-tax return. You want to keep tax inefficient assets out of taxable accounts, and you want to keep the asset class with the highest potential for gains within the Roth IRA due to being tax-free upon withdrawal.
My current split is:
Roth IRA: All of the Gold, All of the Stocks, ~1/2 of the Bonds
Tax Deferred: ~1/2 of the Bonds, ~1/2 of the Cash
Taxable: ~1/2 of the Cash
My rationale for this is cash is the asset class with the likelihood to earn the least over the long run. Thus, I keep it out of my Roth IRA. I can still rebalance between asset classes completely within my Tax Sheltered Accounts without any taxable event occurring.
I keep the cash in taxable because of the low interest rates in today's market. I'm penalized by paying taxes on the interest, but the interest rate is low so the taxes are low. If interest rates rise, I can easily "sell" the cash to buy stocks in taxable while simultaneously selling stocks in my Roth IRA to "buy" cash to maintain the correct 4x25 split. However, the reverse is not true. Once I keep any asset other than cash in my taxable account, there's a strong likelihood of a capital gains taxable event to occur if I decide to switch out.
Further, I have access to some specialty vehicles only available in a taxable account such as iBonds and Rewards Checking Accounts, which give a significantly greater than market value return in interest at the expense of tax inefficiency. Also, the iBonds create an additional tax deferred space that is not available otherwise. The overall after-tax return is still much greater than what I have available if I kept the money in a Tax Deferred account.
Finally, my financial life has a lot of uncertainty in it and I may need to live off my portfolio for periods of time. Having cash in the taxable account serves as my "6 months living expenses" buffer, which doesn't require me to liquidate stocks and incur a capital gains event. Because of the uncertainty of employment in my line of work, that is not an "emergency" fund and the likelihood of needing the money is higher than average.
My biggest regret and downside of using this plan is not having enough taxable asset space to purchase gold coins and stocks. Gold coins are superior to ETFs but my taxable space is only around 10% of my total portfolio value because I like to max out my available tax shelters. I'd like to have some stocks in taxable accounts for the option to tax loss harvest in times of market decline. However, my savings rate relative to available tax shelters is such that I am unlikely to grow my taxable account to be much more than 10% of the total portfolio over the next decade so I'm "stuck" with most of my assets in tax shelters

Please critique my strategy and also post yours.