You're technically required to pay interest and a fee to the federal goverment if you underpay by a large enough amount. They never seem to bother to collect, though. Nice one-way road, huh? Then again, I suppose it's my determination as to how much I give them throughout the year. I understand the lack of interest, but it's better for me to make sure that I don't have to pay rather than worry about the few dollars that that interest would get me.