If you only believe you can improve your GMAT score by 20 points, then it is better you spend time on other parts of your application. This being said, if you feel you could increase it by more, remember that a GMAT score is self-reported, so admissions teams will only see your highest score when you apply. There is no harm in taking the test again if you feel you can improve your score.
Hope this was of help.
Conrad and the Stacy Blackman
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MBA blogger, US News and Author, The MBA Application Roadmap