I've been reading a lot of people posting how they just take the first NBME within a few short weeks of their expected test date and start freaking out about how low they scored. Everyone should be taking the first NBME early on in their studies to see where they stand. The NBMEs are a tool to help diagnose how you will perform on the real thing!
You should use it to focus on your weak spots, then after some more studying, take another one. Repeat again with your weakest distributions, and then take another one. Repeat the procedure. By the time you've taken all of them, hopefully some time before you take the real USMLE, you should have a strong understanding of exactly where you stand, what you need to do, and whether to postpone or take the exam.
So please! Take the NBMEs seriously as a learning tool, and not as a last minute evaluation!