Do you use margin and if so how much?

Margin is such an interesting subject. If the interest rate of a margin loan is below the average return of the S&P500 it seems smart to use at least some margin. Obviously not an amount that quickly eats away at the underlying capital but a conservative amount like 10-20% seems like a very easy way to pad a portfolio.

I know there is also a popular ‘rich people’ strategy of buy, borrow, and die so that you never have to face the capital gains tax and can get a solid step up basis for the next generation. But even beyond that it seems smart if you are simply using the margin funds to buy more of the underlying asset, especially with the tax advantages that come with it.