You don't need to replace the brake lines at the same time that you replace the pads and calipers. If you don't have any braking issues now (other than the warped rotor) they you won't have any issues after replacing pads and rotors. As a matter of fact, your braking may get better with the new stuff.

Like Tom said, if you're going to open up the system, then by all means do the lines at the same time, but you don't need to open the system in order to replace pads and rotors. I went to slotted and drilled rotors many years ago, and since I wanted to paint my calipers to, I went ahead and installed the stainless steel lines at the same time. There are quite a few here who are lifted with large tires who still have the stock rubber lines (Jo was one of them for a while, I think) and they are doing just fine. Just check out the existing lines while your there installing the pads and rotors to make sure they look good and aren't cracked and you should be good to so.