Tile is no different than a wood floor or any other assembly of pieces on a surface. Start with the center of the room. Identify how square the room is. The accuracy of the how square the room is relates to the overall dimension consistency of the room. Measure the width of the room at one end, then move down to the other. Repeat this procedure in the other direction. If the measurements are consistent, that means you have a room that is square, or has four ninety degree corners. If this is the case, the layout is easy. If the room has some variation of this perfect square, then a decision has to be made. What is the least obvious wall to make up the difference in dimensions? Is there a wall that will be covered by furniture and will not be obvious to the individuals in the room? If there is such a wall, then this is the area that the dimensions will need to be adjusted. However, if the room does not have a hidden exposure, then the dimension difference will need to be split, to accommodate the out of square room.
Decide on the width of the joints for your tile installation and measure the actual tile. I know some tile installers that actually take a length of wood, and lay the tiles with the proper spacing, alongside the length of wood. Once this is done, they mark the wood with a sharpie to indicate each piece of tile as well as accommodating the width of the joint. With this guide as a template, the layout of the tile is made, extending from the center of the room. The basic rule, if there is not an area that will allow the adjustment of inconsistencies with the accuracy of the room, to split the differences in the dimensions. It is important that one side of the room does not have a sliver of a tile and the other side has a full tile. Split the difference and the field of tile will look consistent and accurate.
The other method of accommodating dimensions that are not perfectly accurate is to open the width of the joints between the tiles to accommodate the dimensions. For example if the intended joints are designed to be 1/8 inch and, if the joints are made 3/16, the accuracy of the layout is accommodated, the spreading of the joints may correct the inaccuracies in the room. Another trick is to accommodate the dimensional difference, with a larger piece of base trim on the wall surface, to cover the inaccuracies. There are several different methods of accomplishing an accurate tile layout. Creativity is the general rule, and there is no set and fast rule, regarding the layout of tile in a room that is not square.