They're going to see it anyway if you retake, and if you cancel, they'll see that you cancelled the score. Prepare as best you can, stop worrying about it...stay focused on what's important so that you aren't distracted during your test.
If you bomb but retake and do better, no worries. They look at your highest score. But if you cancel before you see it, you'll never know. However, if you KNOW you're bombing during the test (I did that once when I was sick), then cancel. Otherwise, bite the bullet.
Totally useless response, eh?