The Official Guidelines
The actual guidelines established by the various tile agencies, such as the National Tile Contractors Association and the Tile Council of North America, are constantly being revised to take into account modern advancements in the industry. The guidelines for grout joint width that are established are nothing more than a frame of reference. The official wording is that for every 1/16-inch variation in face size, the grout joint should be three times the variation. For example, if there is a 1/16-inch variation in size between two 12-inch tiles of the same size, a 3/16-inch joint should be used to hide the variation between the two tiles.
With certain tiles, such as polished marble and granite or honed travertine, the edges of the tiles are beveled at the factory. While you can install the tiles relatively tight to one another (adjusting only slightly to make up for variations in the size), you will always have at least 1/8-inch of exposed area if you decide to grout the installation, given the fact that the beveled edges are around 1/16-inch on each edge. When butted against each other, there is still a 1/8-inch beveled valley between the tiles.
Tight installations are possible so long as you follow proper installation methods to allow for movement of the installation over time, such as with foot traffic or seasonal shrinkage and growth. However, regardless if you are working with natural stones or man-made tiles, there will always be some type of variation in size, which means that while you can install the tiles relatively tight, it is an impossibility to have every single tile tight to each other. There will always be some small gaps throughout, creating tiny voids between tiles.
Natural Stone Installations
Natural stone tiles are quarried rather than manufactured, which means the size variation is always greater than with man-made tiles such as ceramic or porcelain. While you may want an extremely tight joint with natural stones, you have to take into account that some stones, such as tumbled marble, slate or others, can have drastic differences. It is not uncommon, for example, to find slate tiles that vary by 3/16-inch or more in a batch of 12-by-12-inch tiles. As such, you have to adjust the size of the grout joint to accommodate for these variances.