In all the time i have tiled i have almost always used spacers on walls and rarely on floors.Try using rubi long legged spacers, have never had plastic visible once grouted.
I agree kap, how often do you get floor tiles that are all exactly the same size, layed 16m2 kitchen floor this afternoon in 2 hours including all cuts and cutting door facings, went out to van had a tidy out couple of **** went back in grouted, job done, not a spacer in sight!!!!:shades_smile: