The GMAT actually does penalize significantly for unanswered questions. While your score is pretty much determined by the end of the test (due to the computer adaptivity of the exam,) what we've heard from the GMAC is that for every question you leave blank, the test penalizes you 2-3 percentage points (they haven't told us exactly how much.) So, if you were at a 99th percentile on the second-to-last question, but then left that last question blank, you would end up with a 96th or 97th percentile overall score!
There are some good tips for this in our guide: the GMAT Uncovered: http://www.manhattangmat.com/gmat-uncovered.cfm
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Caitlin Clay | Manhattan GMAT Student Services Associate | New York
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