The goal should be to avoid small slivers of tile on the edges of the whole wall. Then, the fixtures get installed where they need to be. It is possible to make a hole anywhere in the tile - it doesn't have to be on an edge; it just takes more skill and/or tools to put one in the middle of the tile. It's often more important to get the centerline of the fixture (shower head, control valve, etc) centered or positioned where you want it left-right and up-down on the wall, disregarding the position on a tile.
You can get more opinions and help on tiling at www.johnbridge.com Tile Your World website.