Installing accent tiles is probably one of the best things you can do to add visual interest to the shower. A vast expanse of tile can easily look boring, plain and somewhat cheap, but simply adding a row or three of different tiles gives it a custom look. There is no standard height for accent tiles, and the best place for them depends upon your shower. It's helpful to sketch your design before you begin installation so you can play around with different heights without making a mess of mastic.
Using accent tile in the bottom row "grounds" the wall and makes it look finished in the same way baseboards do on living room walls. If the shower floor is especially decorative, the accent tiles act as a frame, drawing the eye downward toward a lovely mosaic or pebble floor. In some showers, this may be enough decoration. If your tile is very busy or ornate, adding additional accent rows may clutter it.
Chair Rail Height
Living room chair rails may be anywhere from 24 to 36 inches off the ground, and adding accent tile at this height allows the continuation of a familiar design element that works especially well if you have a chair rail or wainscotting in the room just outside of the bathroom. There's no need to be precise -- anywhere between thigh level and waist level is fair game. If your ceiling is very low, keep this row of accents toward the low end of the range to visually heighten the room. Conversely, if your ceiling is very high, putting the accent row at waist-height will draw the eye upward.
Picture Rail Height
Like the Colonial picture rails in decorator's magazines, a higher row of accent tiles serves to add another line of visual interest and anchor the wall more firmly to the rest of the room. Picture rails are generally about 12 to 20 inches from the ceiling -- keep it toward the higher side if you're using an accent row at chair rail height as well. If the two rows are close together, it visually shortens the shower space. Picture rail height rows can be adjusted up and down to emphasize ceiling height just like chair rail rows.
Accent tiles in the top row echo the look of crown molding. They visually lower the ceiling, and when used only with accent tiles in the bottom row, serve as a frame for the space. Avoid putting accent tiles in the top row if you're using a high picture rail height accent row, unless those are the only two accent rows in the shower. Otherwise, the wall becomes cluttered. If your shower is especially small, avoid using accents in the top row to keep from calling attention to a very low ceiling.