Are grout lines or gaps required for expansion with ceramic tile?
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&amp;ie=UTF8&amp;qid=1450267844&amp;sr=1-1&amp;keywords=setting+tile