It depends on your body and your motion. Generally speaking, your back foot should fly up above your butt after you release the ball. From there, ideally, it will follow your forward momentum and end up close to your front foot, with both feet at about shoulder-width apart and facing home plate --- putting you in a position to field a ball hit back to you.
That's ideal --- but everyone is different. Again, I say generally, you should at least have some control of your body. But there are always exceptions (Dontrelle Willis, Oliver Perez, Mitch Williams, to name a few). Unless you're throwing in the high 90s and able to control your pitches at that speed, it's probably a good plan to work toward a controlled follow-through.