Back in the days before the IRS Underpayment Penalty, every year, my Dad would file for exemptions until they ran out and still not pay until after he received the Third demand letter. He would have to pay interest, but No Penalty. One year I didn't have the funds to pay on the 15th, but by the time I received the 3rd demand letter I had scratched up the money and sent it in. Interest I had to pay was not too bad.