Ok I wrench for a living so I will give you the work answer then I will give you the way I do my cars.
As many have stated you can "mic" the rotors and see what you have left. If you figure the amount of wear you inflicted on the rotors and that number is less than half of the wear, (from origional to min. thickness) keep your rotors and change them with the next set. If you don't have any pulsations when braking you shouldn't need to resurface them.
On My cars, I change the rotors every time I change the pads. I do this for several reasons,
1 I am very hard ( And I do mean very) on my brakes, I love to set the car late on entry, and drive alot of country roads
2 I run Hawk HP+ pads. They will kill a rotor.
