I am by no means an expert on this process so I really can only speak from my own experience and how I felt about the one test I have taken (which I managed to do reasonably well on - for me at least).
I think it would be a mistake to test before completing the prereqs. I am sure this is very common, but top notch scores are not very common. Personally I'd rather do what the top 10% of scorers do than what "most people" do. Not only will this dramatically streamline and facilitate your prep for the exam (it is much easier to review than to learn for the first time), it will give you a sort of knowledge base safety net. Even if you didn't explicitly study the concept behind some off the wall question, you are significantly more likely to have some clue how to approach it if you have spent time listening to a prof three times a week, studying for exams, and working out homework problems on top of studying for the MCAT as opposed to just the latter. I think this safety net would be particularly valuable on the PS section, which is relevant to your situation.
Naturally, I think this depends on your own ability to perform well in a relatively easy class like university physics. If you don't totally blow it off and actually learn something, it can be a tremendous asset to your MCAT endeavors.
A final word of advice: don't psych yourself out about the MCAT. Other premeds and certainly the ravenous, cash-hungry test prep companies do a great job of generating a palpable atmosphere of MCAT terror. Study hard and, more importantly, study smart and you will earn an excellent score. Just my two cents.