I don;t think davelj is suggesting that you put zero down. Just use zero down interest only (at whatever rate you get) to determine whether it is worth doing. This removes the need to figure opportunity cost.

This reasonable and is equivalent to assuming you would get a rate of return equal to the mortgage interest in computing opportunity cost.

Of course you get the best rates for 20% down or more (30% for investment property), so you have to weigh the benefit of lower rates against the benefit of having less skin in the game in case of substantially more downside in the market.