Never used a spacer on floors,
On walls I use pegs or crosses, always pull them out. I've seen a few rooms tiled with the old tiles with lugs on that have bellied in the middle
As most of my work is rectified porcelain I use spacers on the face including floors, but with any other type of floor tile I do it by eye and a tape measure.....
Most of our clients over here prefer a minimum size joint about 1.5mm to 3mm max. I prefer to strike a chalk line and run a straightedge square to that then run in any direction using spacers to keep it all square and straight. With large format tiles you dont have too many spacers to pull out.