Better to be safe than sorry later. The expense of purchasing a rotor now might save you a much larger expense later.

I was talking to some guy at Pineapople racing when I was doing my rebuild. He said if it's not too bad you can sometimes reuse the rotor. Mine was F@#KED UP, unfortunately. The ding might not be so bad but the groove would worry me. I am no expert however. Just an opinion.

toast, toss it...or use it as a bookend