It's better to register for the exam now and hold yourself accountable. Once you have a date set, you have no excuses to diverge from your study schedule. Further, once you tell others when you're taking the exam, it social motivates you from the outside to uphold your promise of taking the GMAT exam and scoring well. Don't under-estimate this as one of the biggest motivations for you to score high.
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What's Inside GMAT Pill?
Zeke Lee, GMAT Pill Study Method (Study Less. Score More.)
GMAT Pill Reviews | GMAT PILL Free Practice Test