 If the taxpayers don't pay the 90% and they aren't protected by so-called safe harbors described below, they can owe a penalty that's an interest charge on the underpayment. The rate is set and published quarterly by the IRS. Currently, it's 8%. So making a payment to the IRS at any point can reduce this interest. So filers facing these chargers can save by making payments before their April 15th due date.