In spite of what you might have heard, standing right in front of Cinderella's Castle in the Magic Kingdom is not the best place to view Wishes! If you stand right in front of the castle, you will be looking almost straight up and will miss a lot of the fireworks.
As you can see in the shot I took from the side of the castle, over near Big Thunder Mountain, the fireworks are far behind the castle! In order to see the full magnitude and beauty of the fireworks, the best viewing area, in my humble opinion, is right as Main Street opens up into the hub. You will be close enough to see the majesty of the castle, while being far enough back to see the magnitude of the fireworks. My absolute favorite spot to watch Wishes from?!?!? Hmmmm....that's a secret that I share with my clients! :)