I think it depends on the school. One or two of them don't even ask you to report the writing score.
That said, the key to the GRE essays is to look for the logical fallacy in the argument put forth. If you can write persuasively about why the argument is illogical, you'll do fine. Also, be aware of time. Take 5 minutes to think about it, 5 minutes to outline, and then write.
I hope you get a good night's sleep, a good breakfast, and then relax into it.