I would recommend always filing your tax forms--The IRS requires any non profit organization registered as a 501c3 to file 990's if their GROSS amount exceeded $25,000..
Having a tax exempt or tax id number doesn't necessarily mean that your PTO is a registered non-profit 501c3 organization. You might want to start by calling the IRS at 1-800-829-8815 to see if your PTO has ever filed the application. If not, you might want to consider completing the process.
To the best of my knowledge, our PTO has not filed income tax forms. We have a tax exempt # and also have a "large sum" of money as well as a large CD account. Should we be filing or filling out forms?