If you apply to schools which require the GMAT, then you have to give them your GMAT score; they won't accept your application otherwise. And if you give GMAC permission to share your score with a school, that school will see ALL of your scores for the previous 5 years. So if you don't like your first test, and you take it again and "send" those scores (you're not really sending anything - you're just giving the schools permission to access your scores), then they will see both tests anyway.

Your questions seems to indicate that you can choose to show a GRE score instead - check with the school on this, because not many b-schools will accept the GRE in place of the GMAT. Of course, if your school does accept the GRE instead, then yes, you might not want to select schools before the test. Just be aware that you will have to pay $28 per school after the test if you do decide you want to send the scores. (You can pick up to 5 schools for free before the test.)