You won't have to report it, but if you're like some people who go ahead and complete/submit the AMCAS with the expected MCAT date, you'll have to explain why there isn't actually a score from that date.
Like one poster above said, unless you get violently ill or something dire happens, it's unadvisable to void a score. People usually get that feeling of impending doom when it asks you in black and white if you'd like to void.