if you will not feel comfortable enough with self-studying physics to do whatever you consider to be acceptably well on roughly one-half of one-half of one section of the mcat (where they also give you formulas 90% of the time), then wait. if you can self-study and feel confident about it, or if you've had any exposure at all to the material in the past (like high school physics), take it in may.
i would personally take the mcat in may and submit my apps in june/july. just grab an mcat book and learn the physics section - you don't need to know everything in excruciating detail, just the concepts considered important by the mcat test writers. take a look at a few practice PS sections to gauge what types of E&M questions are favorites.
my rationale is applying early for sanity - if you go back to around september/octoberish on sdn, you'll find lots of anxiety over 'when is applying considered LATE' and other variations on that when early application people were interviewing/being accepted and later application people were waiting to be complete. best to just be early. unless not waiting would result in bombing the mcat. then it's best to wait.
