Take it when everything is fresh in your head. If you did good and learned a lot, you probably don't have to take a prep-course. I think the biggest mistake I made was that I didn't take it as soon as my prereqs were done. Get them done, save two months and study hard, and take it. That way if you have to retake it you will have less pressure.
And don't even apply the physics courses as a criteria for taking the DAT. It isn't tested, so it doesn't matter if you complete it or not. 90% of predental students take physics their senior year, and sometimes after they have already been accepted by numerous schools (some schools require only one semester).
All in all, Desi's answer is probably the best one. Take the exam when you are fresh. Frankly, you could take it having completed bio courses, gen chem courses, and only one semester of organic chemistry.