Not really, rotors can warp nearly the first day you drive it, but its nearly undetectable. The warping over time increases as the heat/cool cycle applied to the rotor goes on with time. warping can become an issue when it creates gaps where the pads don't touch, or touch in a limited way. This causes pitting and rust to build up and increases the above alot, there by increasing the pitting. Warping of rotors can happen really quickly if you don't drive vehicle for an extended period and rust develops, if your wheels are not torqued down correctly if you have aluminum rims, or you drive in a lot of stop and go traffic. And of course the famous metal on metal when brake pads are worn away.
Turning down rotors trues them back up but also takes a small amount of material with them, in turn you may actually warp your rotors faster over the next 20k as there would have been wear anyway at that point. You always put new pads on when you resurface the rotors, or you wasting time and money. For about the last decade I just put new rotors once the pads wear out twice although I am a not expecting to get a lot of life on the second of pads before warping gets bad. I also don't cut rotors anymore as they are often at or near the bottom end of specs for cutting, not to mention its often easier to put new rotors on.
Chris