The middle is right out. Consider the spine, and how the two spines would have to connect: There would have to be some sort of three-way vertebra (horse back, horse front, human). I've never heard of a split spine in any creature, so we have nothing to base this off. (There are mutant snakes with two heads, but these x-rays seems to show either a blob of bony material, which would be very heavy, or no proper connection at all, which would provide no support.) As one of the other answers mentioned, this would cause the horse spine to sag as the weight won't be supported directly over the legs. On the other hand, having the torso closer to the center of gravity might be more practical for maneuvering. (Semi-related: My favorite mythical creature, the Nuckelavee, has its torso in the middle. It also has no skin and breathes the plague, so it's not a great example of realism.)
What about in the back? It would look absurd, for one. But internal organs are the big problem. There would be no practical mouth, unless you have a) a giant freaky gaping hole where the horse head should go, b) a horse head, which makes it not really a centaur, or c) use the human mouth, which requires a uselessly long esophagus running down its back along the spine, or reverse the internal organs front-to-back, which is just a centaur walking backwards. Additionally, the pelvis of the horse seems ill-equipped to support the weight of the human torso. Just look at those scrawny little coxa. More importantly, I can't see a good way to connect the human spine to the horse spine. The horse spine curves downward into the tail (which you would also have to find a way to deal with) but it needs to slope upward into the human spine and I think it needs more space to do that than the tiny space you can see in the picture, whereas the horse neck is already basically straight up.
One other consideration for the middle and the back: What do you do with the horse head? With the front it's obvious that the human torso replaces the horse neck, but what else could you do with it? Just truncate the spine at the shoulder and have a rounded blob in front? That removes the need for several muscle groups, and probably a few bones. Keep the horse head? Like I said, not really a centaur anymore. You also have the problem of which brain is in charge of what. So torso in front is really the only sensible solution.