NOT picking a spot indicates a lack of concentration on that aspect of the shot. Lack of concentration usually isn't a problem at the target butt, but is an evil thing when hunting. It can show up in many forms, and n-p-a-s is just one of them. If it happens it usually indicates a focus directed more on the antlers, deer in general, excitement, etc. To an extent I believe you have to x out the excited hunter and become more of a focus on the shot person. That's actually easier to do if you're not pounding with adrenaline and desire. The more calm and rational, the better your focus will be. When it comes to live game, I spend hours and hours looking at every single animal I see and deciding where I would want the arrow to impact. I pick multiple spots and practice seeing them to the exclusion of the rest of the animal. My 2 dogs, every single deer, a running groundhog, all the critters on Animal Planet...I zero in on THE spot. When a non-target deer walks by, I pick an exact point of impact and stare a hole through the animal. All this practice makes it a lot easier to zone in on the critical vital hit I desire. I'm far from perfect, but better than I used to be by far.
Incidentally: For me, my problem is watching a deer's eyes. I judge so much by what that head and eyes tell me regarding the deer's alertness and such. I've never been a guy to focus on antlers at close range. I am many times watching a deer's eyes as it enters the shot zone; then I very quickly shift focus to the impact spot and 'zoom in' visually trying to exclude everything else. It works.