Observations from my experience with the SAT: Having a human grade a practice test or grading it yourself doesn't matter. The most important thing is becoming as familiar with the tests as you can. When I took the SAT, I didn't even have to read the directions - this increases not only time available to spend on actual questions, but confidence as well.

Of course, any time that there's an essay portion, the human grading aspect becomes, I'd imagine, way more important. I have no idea if it's worth the money, though. Is there any way you could get a teacher or mentor to review some of your practice essays? Also, in Advanced Placement and SAT II test prep books that I've seen, there are sample essays and a breakdown of their strengths and weaknesses, and if there's a book like that for the GRE that you haven't looked at you might want to.

I got a perfect verbal score on the SAT and I consider myself a writer, but I didn't generally do great on standardized (AP and SAT II) essays. So I think that even with a good verbal score and being a writer, if you haven't gotten any feedback yet about practice GRE essays you've done it would probably be a good idea to do so.