Well, placing it in the center of the room is not always bad. It depends on your room dimensions, really. If your room is very small, it might not matter that much. For larger rooms, there tends to be an strong acoustic “bass void” in the center of the wall at many frequencies.
We know that many of our customers place the subwoofer where they like, even if we recommend otherwise. Usually there is no real harm done.
However, we know that the room itself is extremely critical to achieve powerful, accurate sound. The room is often called the final speaker, component, etc. to bring home the point that the room setup makes much more difference than most speaker cable or other tweaks. That is why we personally pay attention to subwoofer placement.