I don't think there is a hard and fast rule. If you feel like taking an exam again as practice, do that. If you want to work through your old test results and review the areas where you got things wrong, do that. I would suggest that you do both if you have enough time. If you've studied more and feel like you would do better on a test if you took it again, start with taking the full exam again. It's your study plan, so you know best what is going to work for you.