There are a number of issues to consider when making this decision.
1. How low is "low"? The average MCAT score of successful applicants is around 30. If your first score was 1 or 2 points below this, your application should be fine, especially if you are retaking the exam. If you scored below the national average (25), I'd definitely consider holding off on applying this cycle all together. You may not have enough time to complete a solid application and study enough to significantly improve your score.
2. Are you being honest with yourself? Based on the full length practice tests I took, my MCAT score was exactly what I expected it to be. You should be absolutely confident about your score going into the exam, especially if you plan on submitting your scores late. A second poor score will be detrimental to your application. One low MCAT score could be a fluke or explained in interviews, two or more cannot. In my opinion, taking the MCAT more than twice looks bad. Scores are valid for three years, so schools will see all the exams you've taken within that window. Use your test opportunities wisely!
3. Its important to remember that the MCAT is only one component of the entire application. Its always important to put your best foot forward, but if your exam scores are on the lower end of acceptable, you need to make sure that you have stellar recommendations, extracurriculars and a personal statement.
