After reading chemprof on handwriting, I caught a thread on collegeconfidential that asked, How much does handwriting affect SAT essay grade?
I would say probably yes, because I agree with this comment:
Officially, it's like jgraider says. They're professional graders, handwriting quality is not one of the things being judged, so it has no effect.
Unofficially, essay grading is by its nature subjective. An essay where all the letters are beautiful and flowing will look better than an essay of chicken scratching, whether or not it's officially used as a criterion.