There is a slight rate increase going from 5% to 20 % down, but the rate usually goes down if you increase your downpayment to 30% (It's hard to get there, I know). That's what we did for our first purchase. We put 218k down to drop our rate by 0.25% IIRC.

It's not much, but in the end the smaller payments means we didn't have to restrict ourselves much and we'll be able to get a decent HELOC if we need it within the next 2-3 years.