My general rule is to replace, rather than pay the money to resurface.
Most manufacturers, do however state a minimum thickness of the rotors. They usually suggest replacing at that point. I doubt 39K miles with one set of pads will get your rotors to that point.
To that point, if your brakes are not warped (vibration when braking), then I see no point in taking extra material from the discs.
To the flipside, if your rotors are warped, the defects are usually deeper than the surface. Resurfacing/ grinding the rotors will make them dimensionally even, but physically they are likely to result in shaking when braking again. They are cheap enough and last long enough that I don't spending for my safety if I'm in doubt.