Convention is the middle, or 1/2 tile off centre of the middle depending on what gives the best (biggest) cut to the edge to allow for out of square or plumb corners (if walls). However, depends what the room is and the focal points. If a kitchen for eg. I consider what the cuts will look like along the front of the units, door openings etc. and don’t worry too much if there’s a small cut to an edge you can’t really notice. Hallways, again stand at the focal points and look to see what will look the best, visuals are better than mathematicaly correct normally. In Bathrooms, as tiles are getting so large, you need to consider symmetry and layout of other bathroom items, rather than just fitting to the area, for eg. 1200 x 900 shower tray might look best starting from the corner (if plumb) when using 600mm tiles, not so much with 450’s.
BTW, I make my living tiling, so hope I know something about the subject.