Underpayment tax penalty in 2007 and 2008 estimated tax??

We had a one-time situation for our 2007 taxes which led to an underpayment tax penalty and we were given forms for our 2008 estimated tax payments (above our normal deductions). If we know for a fact that we will not be in the same situation for 2008, can we just not make those estimated payments? Are they just guides or will the IRS actually expect those estimated payments and penalize us if we do not?