I try to aim a little low but not low enough to miss the vitals if they don't react. Like was said befor there are many factors, alertness of the deer, other sounds in the woods, how sound is traveling at the time of the shot, bow/arrow noise,....
However, one thing that many people don't understand is if you are shooting at a deer at 15yds and under the best way to keep them from ducking is to not shoot 6 to 12 inches high to begin with. I have done the calculations useing an arrow speed of 180fps, and supper fast 0.1 sec reaction time for the deer(a human reaction time is 0.2-0.3sec) and assuming reaction as soon as arrow is shot and free fall for the deer for max drop distance. I got the following max drop distance: 10yds = o.94in, 15yds = 4.3in, and 20yds = 10.1in. Of corse after 20yds they have plenty of time to do what they want.
ps. a 280fps bow would be: 10yds = 0in, 15yds = 0.7in, 20yds = 2.3in, and 25yds = 5.5in, 30yds = 9.3in (of course the slowing down of the arrow would effect the numbers for the longer yardage a good bit).