Once you get to an LTV (loan to value) where you are confident that if you needed to sell your house you would not be upside down you should be putting as little towards principal as you can and still afford your monthly payment. If you started with 20% down and believe in the market you bought in interest only loans make sense to me. Mortgage rates are generally lower than investment returns AND the government subsidizes your payment through a huge tax break that will probably never go away.

I don't know why anyone would ever pay off their mortgage more than the bank requires them too. Often people argue that if the price of your home falls and you haven't paid down your mortgage you might owe more than the house is worth. Well, if the value of your house is falling why would you invest in it? You could instead put the extra money you planned to put towards the principal in a savings account to help offset any losses. Alternatively, if the value of the house goes up you've made money using someone else's money. It's called leverage.

The folks that can't grasp this concept are missing out on potential wealth creation because they think they are playing it smart and safe. In fact, they are playing it dumb and risky.