The PCV valve has little to no affect on gas mileage.

It is easy to get to and swap out (assuming the hoses haven't become brittle and fused to it or the manifold) but it only needs to be done if it doesn't function properly anymore. Clamp the hose with pliers while the engine is running and if you hear a click, it is fine. Otherwise, it needs to be replaced.

The PCV valve only needs to be inspected every 60k miles, according to the maintenence schedule.