A deer's front shoulder covers about 1/2 of the heart and when selecting where to shoot, I aim for the 2:00 position of the front leg - placing the crosshairs straight up the rear portion of the front leg and squeezing the trigger when they are about 1" below the top of the shoulder.
Aim for the heart. If you're a little off, you still hit the lungs, anyway. Also, if the deer jumps the string, better chance of a hit if you aim for the heart. (imo)
Dave has a excellent point, however, I always aim for lungs but aim low in case the deer ducks at the release, actually they jump but they will be crouching when the arrow arrives unless its a long shot or they catch movement of your draw.
I could see the small band of mule deer picking their way towards me through the sagebrush 800 plus yards away. With my 10 power binoculars I spotted a legal buck bringing up the tail. I dropped down and crouched behind a large chunk of sage just waiting for them to pass within range. Unfortunately, they stopped and began milling around before finally settling down, with a few of them laying down right out in the open. They were not going to come to me; if I wanted any closer, I'd...