Are you feeling a pulsation in the steering wheel when braking? If not then don't bother turning them unless they have developed a substantial ridge on the outer edge. Personally I use some nice course sandpaper to "rough things up" and remove that glazed look they get sometimes. Lets the pads break in quicker IMHO. I am of the opinion that "turning" a rotor oftentimes leaves you with a thinner rotor that in turn heats up quicker and warps faster.