Definitely don't wait until the last week to take one. NBME exams are good diagnostic tools to uncover weak areas, so take one (or two or three) while you still have time to push hard and to read up on trouble areas. If you're not someone who would get psyched out by a low predictive score during your last week, taking one several days before the actual exam may help you focus your last days of studying. If you are easily discouraged by numbers, I would take the last one a couple weeks before the real thing so that you can probably bet that your score will improve from the last NBME.
