Break them in properly
I too would recommend getting new pads. Using old pads, you run the risk of squeeling, and prematurely wearing out the rotors, due to scores on the pads.
Also, equally important is to set the pads in properly. After they're installed, take time to break the pads in. Go to a nice open road, get up to 40-50mph, then come to a near stop, with steady even pressure. Continue this process for a few miles. Doing this will help seat the pads to the new rotors, and ensure a nice long squeel free brake setup.