Well here's how I do it.
1. Mark a straight line around the room at any height - usually half way. This line is the datum line ( or reference point )
2. Find the highest ceiling point & lowest floor point - First to the ceiling - use a staff and go around the room at 2 ft intervals, marking the staff where it meets the line at every point. You will end up with a series of marks on the staff -the distance between lowest & highest is the amount that your ceiling runs out. Repeat for the floor.
Knowing this will enable you to make sure that the cuts at ceiling & floor do not end up as sliver cuts.
3. Make a staff with tile & grout lines for vertical set up.
4. Choose a point where you would like to start tiling.
5. Mark staff against datum line from this point & go around the room checking what size of cuts there will be at door, window, shower, ceiling & floor height.
6. If you are not happy with any then adjust accordingly until you are happy. You will then know ( & be able to explain to the customer ) - why you have started there. It is always nice to start full tile around the bath - but not at the expense of having, for example a tiny raked cut at ceiling height.
7. Use staff also to check horizontal cuts.
Start tiling in the knowledge that there will be no nasty surprises along the way.