Well,
I just have my own theory about tests. I just did, and recommend, a whole test in every of my last 4 days (2 Powerpreprs and 2 GMATPreps). I also took one challenge each of those days and even one the same day as GMAT (I had GMAT at 16:00). The idea was to feel like the exam and to have your mind as used as possible to GMAT's feeling and, especially, be used to time and nerves management. The main problem is that a bad score can drive you down your self confidence, but that's a risk that it's worth it (at least it was for me, 760).
But I guess I've been always one of those guys who must study until a few minutes before the test. And well, when I finished the test, about 20:30, I was exhausted.
Good luck.