I feel you should take GMATPrep early rather than waiting for your actual GMAT. Here is why..
1) There are two tests in total. both these are official tests with official questions, resulting in more realistic estimate.
2) if you take it day or two before the test, what can you improve at that stage? What is the benefit of knowing how much you will get when you can't change it? It is better to assess your score now then waiting for last day.
3) Some people even try GMATPrep multiple times. Why? because, it contains is lot of real GMAT questions, and practice of real question matters the most when you compare with other unofficial questions. If you wait till last day, you don't get enough time to practice the pool of real questions that you already have with you.