We reposition brackets based on root position as viewed on on the Radiograph as well as marginal ridge heights as viewed in the mouth. On top of that we @drkamsiah also use photos of patients to help us position the brackets.
Ideally, brackets should be checked and repositioned, if necessary twice during treatment. The first repositioning is done after the wire progression is complete (usually about 7 to 10 months into treatment) and the second repositioning is done at the start of finishing (usually about 4 months before removal).