[DesertCat]

Making a larger downpayment that necessary is essentially investing in an illiquid but very safe investment, your mortgage. You will only earn your after tax mortgage rate, i.e. If your interest rate is 6% you might earn 4% because you are losing your tax deduction on that interest (more in Seattle with no state income tax). Essentially its the same as finding a safe 6% taxable investment.

So the question is, can you do better elsewhere? You'll need a second or a refi to tap it later, possibly at much higher rates.

I think its almost always a good idea to increase your down from 10% to 20% because you'll lower the cost of your first, creating net returns on that extra 10% much higher than just the interest rate. But if you aren't getting a lower rate from going beyond that, you just measure it on it's raw interest rate.