Ideally, yes, the drain is in the middle of the shower, but it's possible to place it anywhere. The hassle comes when being offset, the 'short' side to the wall needs to be considerably steeper than the 'long' side. While the shower can also work if you keep the slope the same, that means the 'short' side will be lower than the 'long' side. Most people don't like the way this looks. preferring it to be level all the way around the shower, but it does work okay. Another possibility would be to use a linear or strip drain. Finally, if you prefer it in the middle, you could either reframe, or crack up some concrete and move it.