Grout lines of 1/16 inch work well, provided you use small porcelain tiles. With large tiles, such as those measuring 1 foot by 1 foot, 1/16-inch lines appear too small and get lost beside the large tiles. Tiles up to 8 inches by 8 inches usually pair well with a 1/16-inch grout line. This width of grout can work on a tile floor or between the tiles on a vertical surface, such as a shower wall.
Although it's possible to keep your 1/16-inch line consistent by eye, doing so is usually only advisable if you're experienced with laying tile. Instead, purchase a bag of spacers of the appropriate width to ensure an even grout line throughout your job. Spacers are small and made of plastic or foam. Upon laying a tile, press two or more spacers against the side of the tile and then push the next tile up against the spacers.
Many styles of tile have lugs on two or four sides to help create an even gap for grout. These lugs are made of the same material as the tile and if they're larger than 1/16 inch, this means that by default, your grout line is also larger. It's possible to remove the lugs with a hammer and chisel, although the process is likely to result in a few cracked tiles.
One of the challenges of using a small grout line with a tiling job is that it takes extra care to get all the grout in the gap between the tiles. With a larger grout line, such as 1/4 inch, the grout fills the gap easily as you apply it. Devote enough time to ensuring each of your grout lines get full of grout. Otherwise, the grout will sink as it hardens, leaving you with unsightly holes.