I think it doesn't matter too much if you've gotten a "bad" score the first time you are taking the GMAT. When I took
Manhattan GMAT class, I learned that schools look at the GMAT score you specify in your application. The process goes a little something like this:
You apply to the school X. In your application you state the GMAT score that you want them to consider. Admissions ask an intern to varify your score. An intern looks at your GMAT scores, finds the one you asked to look at, and puts a checkmark as "confirmed." The reason most of the schools do not care about the number of times you took the GMAT, and what other scores you got is because schools care A LOT about raising their median/average GMAT scores to look good in rankings. An excellent way to do it is to report the highest GMAT score of the admitted applicant. Another reason
MGMAT recommends that you do not cancel your score is because a lot of times when you feel like you did horribly, you actually did very well.
I hope this helps.