First of all, the percentile rankings change continuously, based on how candidates are performing over time. This applies to the total score as well as to the AWA score. In fact, as a test taker, it will be very difficult for yo to judge what will be your percentile. As far as an example is concerned, I took my GMAT last month and received a 6.0, which was the 90th percentile. Couple of months before, the percentile corresponding to 6.0 may have been 92 or 94. I hope you got the point.
Further, your statement - "I've heard really good writing will get you a 5.0, and it's nearly impossible to get 6" is not really appropriate. As is evident from my example, when I took the test, 10% of the people scored a 6.0 in the AWA section, meaning it is not that impossible to get that score. Also, you do not necessarily need a very good writing to score a 5.0. Mostly, what you will need is a solid structure for your essay and ability to construct grammatically correct complete sentences, with not much spelling mistakes.
