Rotors should be turned only if there is pulsation in the brake pedal from warpage, if there is deep scoring, or in some cases if there is bad corrosion on the rotor surface... or perhaps a bad ridge when you want to put new pads on. How deep the scoring and how bad the ridge is a judgemental call.
Some people insist on turning rotors ANY time new pads are installed. I'm somewhat more conservative on turning rotors, since you lose "meat" from the rotors each time you have them turned making them that much closer to discard thickness. This "meat" is needed for heat transfer and stopping power. Plus, some machine shops hog off more material than needed when cleaning up your rotor. They don't want to "baby" the process.
95 E320 Cabriolet, 131K