Hi, this is Jen (instructor). Here's the official word from an ETS publication intended for university administrators evaluating GRE scores -- scroll to page 17:
http://www.ets.org/gre/institutionsThe short version:
- "Analytical logic" is more important than mechanics/grammar
- The essay is looked at as a rough draft, so some errors are expected/overlooked
If your writing can be easily understood, I'd put more effort towards improving your argumentation and writing MORE, faster (on any topic) than towards improving spelling/grammar.
Sincerely,
Jen