Do you have to replace rotors when you replace brake pads?

No not necessarily. If they are warped you would have felt a pulsing in the brake peddle. Do you feel a pulse in the brake peddle?

It depends. Do your car shake now when applying the brakes? If they are the front brakes I typically replace the rotors depending on their condition. If there is excessive wear, large grooves, or if they are warped (which would cause shaking when applying the brakes) I will replace them. The rear rotors are not used as much since the weight transfer when applying the brakes is shifted to the front. I will typically go through 2 sets of pads before replacing, again, depending on condition.

Only if they are bad and can't be turned