The only potential downside to this scenario is that if stocks are down such that you are actually selling at a loss in taxable, and you purchase the identical security in your IRA within 30 days of the sale in your taxable account this is a wash sale, so you can’t claim the capital loss on your taxes. Typically you don’t actually lose the tax benefit, ultimately, because the disallowed capital loss is eventually earned because the basis of the replacement shares that you purchased is adjusted up equal to the amount of the disallowed loss. Now, the problem with a wash sale where the loss is in taxable and the replacement shares are in the IRA is that you completely lose the benefit of the tax loss because the adjustment to the basis of the replacement shares doesn’t matter because they’re in a tax protected space (the IRA) where capital gains or losses taxes don’t apply. Does that make sense?
Which is why in the scenario presented, if you’re selling securities at a loss in your taxable account, you should purchase a security in your IRA that is very similar, but not substantially identical, to the security you sold. Then, you don’t create a wash sale, and you can claim the tax loss. Example: sell total stock market index in taxable, buy SP 500 index (and some extended index if you like) in IRA