You would really need to be absolutely certain that your score won't be competitive in order to cancel it. (If you became ill during the exam, for example.)
We would also offer that:
-Schools only care about a student's top score, with very few exceptions
(say, a series of scores in the 400s followed by a 700, in which case they
might ask why it took so many attempts before the student attempted
corrective action). A low score won't hurt a student except for in the
rarest of cases, but it may actually help a student determine how to proceed
if they know that they need to improve in one area vs. another.
-Many high scorers, who did
ultimately score in the 99th percentile, report having been nervous to see
their scores and had at least a moment's thought of canceling them. It's easy to wonder about
the questions that you missed at the end of the test, and because it's
adaptive, if you've done really well you probably have some doubt about a
few of the really-difficult questions at the end of the exam. Students
shouldn't let that doubt void what may very well be a top score.
-Similarly, students often report that they just didn't think that the
questions were hard enough, and therefore feel that they must not have done
well. GMAT question difficulty is based solely on ACTUAL difficulty, and
not at all on PERCEIVED difficulty. Many seemingly-innocent questions have
embedded in them some fatal traps, so just because a question or two may
seem too easy to be the type that an advanced student may face doesn't mean
that it is, in fact, a lower-difficulty question.