If the rotors are below minimum thickness they should be replaced. If the rotors are within spec's it's always recommended to turn the rotors. The old pads have worn grooves into the old rotors so when you put new pads on it'll be an uneven surface which can cause a lot of brake noise. The over all performance of the the brake pad is diminished since it is not touching a smooth surface.The rotors are made to withstand a certain amount of heat with out warping. Once the rotor are below that minimum thickness other problems can occur. The rotors will warp just like a old vinyl record that sat out in the sun. So when BRAKING you'll start to feel a shaking in the steering wheel and essentially it's lengthening your braking distance. Since you have an older model Cadillac this can happen quite easily since the vehicle is very heavy. My suggestion is take the vehicle or if the rotors are off the car, to you mechanic and have them measure them, it's definitely worth the effort.