What I heard several years ago, and have had reaffirmed by several people that should be a little more credible than your average message board tinkerer, is that for some time rotors have been thin enough (weight savings) that there isn't enough meat on them to reliably turn them. I've personally not had good success with the few I've had turned - it seems once the metal experiences the stresses to warp it the first time, it's pretty much done afterward. The local O'Reilly's charges $12 for car/van, I believe, but it's nowhere near worth the headache with the customer for me to try turning anything when new Napa premium rotors that should last the next 60-100,000 miles are $25-40.
My outlook is, just try not to warp them in the first place. After that, replace and learn from your driving mistake or fix the sticky caliper, etc. that may have caused it in the first place.