Right now, the trend for "wood look" tiles on walls = Horizontal application. We are seeing wood grain being applied horizontally. I'm not sure what your tile is/looks like, but that is one trend that is coming through quite well.

My natural instinct is to stack the tiles like "bricks" on the wall...but for something different (if they are NOT wood look tiles) is to run them vertically. Again, this would be more of a modern approach. It all depends on your home, your style and your tile. No point installing a modern tile wall in a home that is trying to be 18th Century. Pics of the tiles and the space (and mention your style) and we will help a little bit more.