Shower cubilcles tend to have adjustability built in to accommodate this. They are fixed to the wall using tracks which allow for walls being out of square and out of plumb.
The main problem will be where the tray fits into the corner, usually this can be rectified chasing in to the plaster on one or both sides. Generally, with a bit of tooing and froing, this works fine. One other word of advice, always make sure the tray is totally wobble free, and bed the sides fully in mastic, don't just rely on the top joint of mastic to stop leaks.