A 5 is perfectly fine. They don't care about the percentiles. The GMAT essays are mostly really there to make sure you're the one who wrote your application essays. They know it's you in the test center, and they can check the writing sample and make sure you really can write. Generally, 4.5+ is considered fine; below that, they make check your essays and / or ask some questions if your application essays are stellar but your impromptu GMAT essays show serious deficiencies.
Also, check out this article for help with the essays:
http://www.manhattangmat.com/blog/index ... no-thanks/