Not achieving a '700' largely depends on the program you hope to get into. For many, the top-15 business schools are the holy grail of the MBA. Breaking 700, definitely helps. But remember - when you are applying to the top program, you are competing against a lot of other 700+ scorers. Admissions will look at other aspects of your application: work experience, undergraduate institution/GPA, amazing essays, etc.
Of course, when you fall below the 700-mark those aspects of your application take on even greater importance. And burnishing your application - vs. spending another two months prepping in the hopes of cracking 700 - will be better a much better use of your time.
As Dimitri mentioned, for programs not in the top-15 a strong application and a mid-600 score can count for more than a 700+. I've known of students who score 750, and assume that they will get into a top-15 program. They put scant effort into their application and often end up in a program in which the median score is 640 (meaning that they are in the same class with sub-600 students, who clearly took the time on their application).
Hope that helps!
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