For best looks, the bottom row of tile should all be the same height. The minimum slope to the drain should be 1/4" per foot. If the drain is the same distance (i.e. centered), then the total drop would be the same, and the tile would be even. When you move the drain off-center, the long distance still needs to be 1/4"/foot, so on the short side, the slope must be greater. If you move the drain too far off center, that slope on the short side can get pretty steep. If you make that side also 1/4"/foot, it will be lower than the long side. That makes tiling it much tougher (use smaller tile) and potentially slipperier (the smaller tile have more grout lines which give you more grip which helps). So, when the shower is not square or if the drain is not centered, you will have different slopes to the short and long sides...you don't want the slope to be too great.
For good help on tiling check out
www.johnbridge.com.
