I remember being in one of my econ classes (game theory) and we had a penalty kick problem/study done that had recorded all the penalties taken in a league since its inauguration (can't remember which one). Point being, you should always choose to kick it to a side (assuming that neither you nor the goalie know which way the other is going). You are always much more likely to score by picking a side, never pick middle. Hope that makes sense but it's kinda hard to explain
Edit: I completely explained that incorrectly. Think about it like this, if I'm the penalty taker and I think the goalie is going left, I go right. If I think he's going right, I go left. If I think he's staying down the middle, I pick a side. It's never to your advantage to pick right down the middle because as say looking at Lukaku's PK, the goalie went one way but still got a foot to it.