My expert advice: when in doubt, cancel your score.
Why? Because you can't cancel your score after the 72-hour window, but you can always reinstate your cancelled score later, for a $50 fee, if the 650 ends up being your best GMAT score. If you don't cancel it, however, and you end up scoring higher on a subsequent GMAT, then any B-schools you apply to will also see the 650 on your score report.
It's not the end of the world to have a lower GMAT score on your school score report, but it does somewhat detract from the strength of your application. Better to keep cancelling until you have the score you really want, and reinstate the score later if necessary. That way, B-schools will only see the one GMAT score that you want them to see.
Remember: although GMAT score cancellations *are* indicated on your personal score report, they *are not* indicated on the score reports sent to schools.
More info:
GMAC website