There are 4 main choices that are mainly a visual preference or may be determined by the position of the plumbing.
 
Against the wall – this allows for the floor to slope gently towards the wall and actually slightly increases the flow capacity as a small head of water can be created.
At the threshold – as the floor will slope towards the threshold a smaller slope is necessary.
In the center – often used if existing plumbing cannot be moved. Floor each side slopes towards the drain.
A fourth option is where multiple units are used either as the shower is a walk through design with one unit at the entrance and exit or if a higher capacity is needed whereby units are positioned against each side wall.