I would only retake if you have a really strong reason to believe you'll do significantly better on the re-take(at least 517+). And by really strong reason an example would be doing far far better on practice tests than you did the first time around.
You gotta realize
a) A good number of schools average multiple MCAT scores from an applicant. And whether a school does or doesn't isn't really that relevant; it will be noted there were multiple MCAT attempts on your record which will factor into a judgment on your app, regardless of whether a school has an official average multiple score policy.
b) Re-taking and doing the same hurts you. Retaking and doing worse definitely hurts you and can start messing with your chances in general.
c) AAMC's site changed so I can't find the official re-take stats but only 1/3 of people or so who re-take a 30-32 get 3+ points better which is what you need. So based off history 2/3 applicants like you honestly probably hurt themselves re-taking.
d) Top schools get enough applicants who hit 35+ on the first attempt they can be picky about applicants who need multiple attempts to get that score(and this is making the big assumption you hit that improvement).
e) Many ADCOMs are not big fans of people re-taking perfectly fine MCAT scores. It telegraphs something to them which is not particularly favorable for an applicant. From what ADCOMs such as gyngyn and Law2Doc have said in the past, there are definitely some ADCOMs and schools who won't really be huge fans of re-taking a 32, even if the re-take score is definitely better.
So all in all I'd be somewhat cautious/careful about a re-take. Doing it just to gun for a top 20 school is not the most rational or wisest of decisions IMO.
