Tile pattern for a shower tray

Hi I am about to start tiling my entire bathroom which will have a shower tray installed. I would like to know if I should use the top of the shower tray as a baseline across the entire wall and work up from this adding a thin tile section below this line or should I still find the vertical centre for the whole wall and if necessary cut a tile that sits on the floor but goes above the top of the shower tray which would mean the bottom row of tiles in the tray itself are much shorter in height to match up. This is presuming I fit my tray before tiling the walls.