But be sure you've at least explored the idea of incorporating before completing your forms. It's not transferrable. If you get your 501(c)(3) designation and incorporate later, the incorporated PTO is a new legal entity. You'd have to submit the paperwork and pay the fee again to get 501(c)(3) status.
Incorporating is a very good idea. In most states it's a pretty simple form and a small fee. (But we've heard from a few that it's more expensive/complex.)