For the grid-in questions there is no need to reduce your fraction unless it won't fit. For the multiple choice questions, answer the question. Before you reduce any fraction, check for your answer in the answer choices. If you don't see your answer, reduce the fraction and check again.
You do realize that your TI-83 or TI-84 calculator will reduce the fraction for you, right? Type in the fraction as a division problem. For example, "12/36". When you press enter, you will get the decimal equivalent of the fraction. Hit the "math" key. On the screen you will see a numbered list of choices with "1. >fraction" already highlighted. Press "enter" to choose it, then "enter" again to change your previous answer to a fraction. The fraction will be given in its reduced form.