I did a job last year with a tiler buddy of mine of 100sq mtr of 45x45 porcelain on a diamond. These were a direct import from Italy by the owner and as such were supplied by him.
After the tiling was completed we grouted up in a polymer modified grout also supplied by him,won't mention the name because it would be classed as swearing.
All seemed well until it was all buffed and polished. Certain water marks appeared on the surface over several tiles.
No problem we think get the Lithofin book and look up the spot/residue remover,apply,happy days. Well, not quite. After trying 3 different acid based cleaners the manufacturers of the tiles were called in to inspect.
It turns out that certain manufacturers make the same tile in two versions,i.e., One for the home market, One for export. This one was a direct import produced for the home market and as such was not 100% porcelain. The reason for this is that it is cheaper to produce if mixed with a ceramic when it comes to the polishing stage,i.e. being softer,it is less demanding on the polishing wheels than on a fully vitrified tile.
The reps of this tile company agreed that the porcelain was sold as class1 porcelain and agreed to polish the whole area with diamond polishing machines.Cost 1300 euros.
If you go to a tile shop that sells the higher end of Porcelain tiles almost all will recommend that you use a sealer either prior to grouting or fixing because the surface of the tile can take on grout and change its colour particularly lighter porcelain(sandstone,limestone lookalikes,white tiles etc).When they first come out we used to say that you lay porcelain and forget it,i.e. there's no maintenance regime unlike natural stone. If you look at the Fila,Litho,HG books there are loads of products now all designed mainly to fleece the DIYers and boost the ancillary sales to Tiles.
The answer to this is always trial the grout on a patch that cannot be seen first like where the washing machine or fridge sits.If you and the owners are happy then Grout away.Happy days again...