Hi Margaret,
Different schools put different weights on the essays. I've had some students tell me their schools required a 5 or higher, and others who said that their schools said it was OK to just leave it blank because they didn't need it!
I think the best thing for you to do is to contact the schools that interest you; anything else is just guessing.
As far as preparing for the exams, most prep books and prep courses will provide you with a methodological approach for constructing the essays. This helps save precious time, and helps better target what the GMAC folks want.
Best wishes,
Tatiana
Tatiana Becker | GMAT Instructor | Veritas Prep