Some of you may have had good luck just slapping on new pads, but I don't believe that it is good practice.
If your rotors are warped, they have been stressed (or stress relieved) I'd replace them, because they will likely warp again, next time they go through a rapid heating/cooling cycle. The Brembo's I have been using on my Maxima run around 70 bucks blank, about $110 drilled and slotted (each), so it is not a horrible expense. OEM rotors could be higher, discouraging people from replacing them. As a side note, BMW specifies rotor replacement for the M3 ANY time pads are replaced.
More seriously, though, is replacing pads on scored rotors. Pad material is HARD. It does not just conform to the scored pattern for many hundreds, possibly thousands of miles. Because the pads initially touch on only a fraction of the braking surface (only the highest spot) they cannot be properly seated (also called bedding in), plus they can fade almost instantly on a demanding stop (like emergency braking 80 mph to 30mph or so) because the little touch area can't handle the heat build up. Not a good way to find out. I learned this the first (and only) time I just put pads on the front of my (then) '85 Mustang GT. It felt like there was oil on the discs. After I had the discs turned, the same pads worked fine.
Taking the discs off is not a big job, and turning them is inexpensive. It would seem like foregoing a lot of quality-of-repair for minor savings in time or money. Granted, that's only an opinion (mine)
Cheers,
John