On conventional financing you are required to pay PMI on LTV's over 80%. You can choose to pay PMI in the traditional sense, called BPMI (for borrower paid MI) or you can accept a slightly higher interest rate and opt for LPMI (lender paid MI), so it's just charged to you in the form of a higher interest rate. Or you could just opt to do a combo loan, little higher rate on the 2nd but it's not a bad alternative since you can just concentrate on paying extra on the 2nd to eliminate it.
Personally I wouldn't put all 20% down and leave you with little to no reserves... I hate the feeling of being "house rich and cash poor", as it's much easier reaching into your checking account to get money out vs. applying for a home equity loan to get more of your equity out. Plus with interest rates so low, I'd leverage as much of your cash as you feel comfortable with.
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