Hi Billoz,
I think it's a bit late to realise that your walls are not plumb,now that you are ready for tiling.If, as you say they are not miles out,then the finished job might not look too bad.You can only work with what you have.The tiles will have to be put on plumb and horizontal,so anything that is off will show.When you plan your tiling,try to work it out so that anything that is off, is in a part of the room that wont catch your eye.If a wall is just a bit off,then you can put the tiles on, and bed them out a bit towards the corners as you tile up the wall,so as to make it look more pleasing to the eye.No room is ever perfectly square,but if you know in advance that the walls are off,then you can plan to make it as un-noticeable as possible.
Regards:Roughcaster.