It's better to do them after you finish your studying.
The more forms you do the better you get an idea about your score.
If you want to be selective then I refer you to this poll http://www.usmle-forums.com/usmle-step-1-recommended-threads/1028-best-nbme-form-usmle-step-1-a.html
The more forms you do the better you get an idea about your score.
If you want to be selective then I refer you to this poll http://www.usmle-forums.com/usmle-step-1-recommended-threads/1028-best-nbme-form-usmle-step-1-a.html