The larger the tile, the harder it is to make the floor flat and the harder it is to have thinner grout lines. If there is any space, you can force unsanded grout in there, but it is more work to actually get it packed down in that small gap. But, with that large of a tile, it is tough to get 100% coverage when using a small notch for the thinset. If you tap on any tile, do they sound hollow? That will give you an idea if you got good coverage. If not, you may have problems. When the cbu was installed, did he put thinset UNDER it? Did he tape the seams? Did you run the calculation to see if the floor was even suitable for a successful tile installation? You also need a (small) ungrouted gap around the edge of the room for expansion. If the room is quite large, or gets direct sunlight from large windows, it may also require an expansion joint(s) in the field to prevent failure. Some tile are not very flat. If yours are cupped or warped, a small grout line will not be able to hide these defects; in that case, larger grout lines are your friend - they will hide imperfections. But, if they are flat, the finished floor should be flat without lippage between tile. There is an industry standard for that. How bad is the height differences you have?
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Some tile have lugs or spacers and are designed to be butted together - the lugs then allow a space for grout. Intentionally butting tile together without those built-in spacers may lead to major problems. If they are rectified (i.e., carefully sized, square, etc.), and you butt them together, you may find it impossible to get any grout in the gap. A well rectified tile can be set with a very small grout line - something in the order of 1/32" or so, but you must have a gap. Grout helps tie the tile together, hides minor imperfections, and prevents crud from accumulating in that maybe microscopic gap when absent. Installing tile without grout is asking for an unhygenic, potentially smelly mess long-term.