Do I need concrete footings for a free-standing deck
If code calls for footings, you need them for code compliance, at minimum. Typically code-compliant footings increase the odds that the deck remains level over time. 

You will also want them (or more than the minimum) for resisting uplift if you are going to attach a roof (think of it as a sail or kite, when the wind blows.) As such a "mushroom" or pad type footing (where there's a concrete column/post and at the bottom of it there is a larger diameter pad) will help to resist uplift - be sure to use adequate reinforcing steel to hold it all together and tie in your anchor bolts.