I'm not a professional and I've only done wall tiles but..
To get wall tiles ultra flat flat my father showed me how to use a rubber faced block as a kind of straight edge. You spread the adhesive using a notched trowel and position several tiles. Keep the trowel at a constant angle to ensure the adhesive is a constant thickness. To level the tiles you rest the rubber faced block across several tiles and tap the back with the hand. It presses any high spots down. You do it several times vertically, horizontally and diagonally untill happy. On large tiles you try to keep the ends of the block in the center of the tiles. I've never tried it on floor tiles but it's produced very good results on wall tiles or various sizes.
I thought you had to leave a minimium gap for grout or it cracked out. I guess if they don't want them grouted that's not a problem. Google found...
http://www.australiantilecouncil.com.au/tech.html Quote:Rectified products have been manufactured to be able to be fixed with butt joints or touching tile edges ie:- no grout joint. If an installation was to be fixed with butt joints in any particular installation, the fixer must be completely aware of the substrate condition, time span between placing the substrate and tiling, atmospheric condition on site, existence of in- floor heating and all other building material characteristics involved. Structural or any other movement joints must be considered.
Sounds like thermal expansion might be an issue with no grout gaps?
I'm sure an expert will be a long shortly