DrCurious, don't take it if you're not ready and here are the reasons why:
If you take it now, ddn't do well and have to retake in August, you are
very likely to blow it off in August. The reason being that summer days are very, very nice, and you get exasperated, a little cocky about having some experience under the belt, and just plain don't give a damn the second time around.
When I went in for the first time in April, I was very underprepared. I was missing 1/2 of my pre-reqs, but that's another story. When the full-day exam was over, all my energy was drained. I really felt that it was more taxing than a marathon (which I've run before). I was very nervous and aggrevated for the 60-day waiting period for my test scores.
For my second time in August, I knew with very high certainty what score I was going to get on PS and BS. I also knew that for me to beat my practice scores by 1 point on each section, I'd need a miracle. And that to drop 1 point, it also take a Christmas in July. And guess what? On test day, I had no anxiety, everything went very smoothly, and I left feeling alright. After that I relaxed, knowing that the scores are what they are and I've prepared adequately, peddled on with my secondaries, and enjoyed life.
IMHO assuming that you study hard and do plenty of practice tests, for every prereq course that you didn't take, expect to deduct 1 point from an 11 the relevant section. Likewise, add 1 point for every relevant, advanced course (like genetics).
Please don't quote me on that. It's not a rule, but something I've observed based on my own performance.
Good luck!