I only needed 3% when I bought my house.
There is a penalty to not having 20% equity though. It forces you to pay PMI, which is some type of insurance the mortgage company charges you. Once you have 20% equity you can have that removed.
However, if you can put 3% down, then take that remaining 17% and invest it into something that pays higher than the PMI charge, it might be smarter to go that route… unfortunately that is not the case for me, we just didnt have much savings at the time and could only afford 3%…