What practice exams do you mean? If you're referring to the NBME's, there is no set order you have to do them in. All of them are independent estimators of your score. That being said, however, some are more representative than others. For example, I would stay away from the earlier forms in favor of the later forms.
Take 7 as a diagnostic
11 to gauge where you are after half of your study period
12 right before the test (within a week before) to predict performance
Since your 6 weeks out, I would use what I describe above as most people only take 6 weeks to study period (i.e. you're at your diagnostic phase)
