33's will fit stock. So if you only want to do 33's, either keep it stock, or a small level. For 35's, you could still to a level or up to 4" lift. A quality lift and shocks will likely give you a better ride.
Aside from MPG, keep in mind that turning radius will be worse with taller and/or wider tires. Also if you're using it for work, or use your bed a lot, keep in mind that getting things in and out of the bed is going to be more difficult. I used to have an F150 with 6" lift and 35's. It looked great, but I used it for work and began to hate the lift. Also trying to get through a drive through for fast food sucked more if it was tight. Sometimes I would have to turn, back up a bit, and turn again.
I bought mine to tow with, so most likely going to keep it stock height with 33's.