Rick and Carl had the most common sense responses, IMO.
Since rotors are usually made of cast iron, the material varies in hardness and different parts will groove under the same force. So the decision to replace them to get a smooth surface is a very short lived gain.
I would like to see a study of the change of rotor thickness while the pads are showing little wear.
Peter's question references the non-existent rule book of auto repairs.
Pads are pads, and rotors are rotors. For the most part, organic pads are cheapest, metalic more expensive, ceramic usually most expensive.
Why? The act of braking is the act of converting the motion of the car into heat. That's it! Therefore, you want materials that transfer heat away from the brake parts as fast as possible. Organic, slowest, metalic, better (metal transfers heat better than organic stuff), and ceramic, well, I haven't seen enough info yet since ceramic material is an insulator (the space shuttle uses ceramic tiles to protect during Earth re-entry).
Rotors are more difficult to rate because every company out there gets their rotors from a different manufacturer, on each shippment, so it would seem.
Unfortunately, you can't even rely on buying "ACME PRIME GRADE" rotors from "ED's Auto Parts" twice in a row and getting the same thing, since Ed's auto parts upline distributor's supply constantly changes.
In other words, you have to rely on your auto parts place to have the integrity to check their suppliers who change constantly up the supply line somewhere.
Avoid those damn Chinese rotors. They're cheap, but not machined straight.
What to replace depends on just plain old mechanics, just like Rick and Carl were saying. Replace pads when required, replace rotors when required.
Brakes work remarkably well even when things are not perfect. That's a good thing. A few miles after you put all new parts on your brakes, they will be wavy, unevenly worn, dusty, and subject to all kinds of contamination from the road and environment.
Therefore, there is some play room between when deciding to replace, turn or maybe just break the glaze off used rotors when changing pads. Leave the decision to a good mechanic who has all the facts.
However, anyone who wants to create a rule like "always replace rotors" either knows something unique about a particular model of car, or just wants an easier, faster brake job for HIM to make more money. There is no universal rule.
The Ole Factory Rep (not Subaru)