And some financial pros even recommend to keep the same asset allocation in all accounts, to simplify rebalancing.
How does that simplify rebalancing? It requires you have transactions in all accounts every time.
Let's say you have $100k in an IRA and $100k in taxable. Your desired asset allocation is 75:25.
IRA: $50k bonds, $50k stocks
Taxable: $100k stocks
Lets say the stock market goes up 20%, bonds stay flat:
IRA: $50k bonds, $60k stocks
Taxable $120k stocks.
Within the IRA, you convert $7.5k from stocks to bonds. Done with a single transaction.
If you did rebalancing per-account, you need to to two transactions.
As a point of reference, between the wife and I we have 8 investment accounts with no real way to consolidate.