If you share you score with a school, all that really means is that once you apply and are admitted, someone in the admissions office will log on to GMAC, punch in your ID, and confirm that the score you reported on your application matches what's on your official test record. GMAC stores your test taker record with them for 5 years.
That's all - they don't get a folder with your name, your score, photo, and your other background information that makes the admissions officer go "hey, check this guy/gal out!" The photo they take on test day is purely a security measure.
Yes, you do have to select 5 schools before you begin the test. And no, there is no perceived advantage or disadvantage to sending your scores to schools before the application deadline. If you opt in for GMASS some schools might put you on a mailing list and e-mail you about events and send you brochures etc. You don't really get to "communicate" with the admissions office on the basis of your GMAT score alone.