I think it really depends on where you are in terms of your prep. If you really, truly do not feel ready, then it would be best to postpone the exam, but not for a long period of time. While it is important to go into the test feeling prepared, it is actually quite difficult to ever feel completely prepared about the exam just because the GMAT is designed to be challenging for all individuals. In the end, the decision will be up to you, but make sure that you leave yourself enough time for a second exam in case you do not do well the first time around and for your b-school applications.
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