As someone who played goalie in soccer for almost 20 years, I will say that rarely is it a pure guess. You try to read the body language of the shooter as he approaches the ball--is he keeping his hips open? Is his foot square to the ball? Laces to the ball?--and then make an educated guess from there. I loved it when guys would try to get all fancy and "psyche me out" because usually they were moving slowly and by the time they approached the ball I had a decent sense of where they were going to go with it. The hardest guys were those that just came up and blasted it, and in those instances, yeah, you really did have to just guess, and shooting straight into the center of the goal would be a good option because no keeper wants to look foolish by just standing there. There are guys that will go straight at the keeper, or even chip it to the center of the net in the event the keeper can restrain his dive enough to manage to keep his feet near the center of the goal.
Your principle is absolutely correct though. There are definitely strategies that could be used that people are afraid to try simply out of potential embarrassment.