Why do I need grout line for expansion ? I have them in the field but on the side wall around a floor vent there is almost no room and the vent will virtually cover the joints.
An article I read by a tile master said grout is for two reasons:
The first is the tiles are not perfectly uniform so the grout helps equalize size mismatch. To check this, put nine tiles in a 3 x 3 pattern tight up against each other and see if there are differences in measurements horizontally, vertically, and diagonally.
The second reason was expansion and slight movement could cause the edges of the tiles to chip or crack the tiles if they are butted right up against each other.
I don't know if I am completely convinced of the second reason since I have seen quite a few tile installations that have no grout lines, especially marble. The first reason makes sense especially with tile with looser dimensions like stone tiles. Otherwise your lines could be uneven.
Here is a link to a really good book by an actual tile master: http://www.amazon.com/Setting-Tile-Revised-Updated-Homebuilding/dp/1561580805/ref=sr_1_1?s=books&ie=UTF8&qid=1450267844&sr=1-1&keywords=setting+tile
I would install tile and all granite and marble with no grout lines at all. I can’t do it. Even though it will look better initially, eventually it will ruin the tile. The best thing to do is use the smallest grout line your particular tile will allow and get a grout that closely matches the tile.