The overall width of the backsplash is important if you have a kitchen countertop and backsplash area that does not work out full tile from the outside edges to the inside corners. Straight-lay patterns can generally use a cut in the corners, but diagonal patterns or straight-lay tiles where a sliver cut is a possibility need to be centered to ensure that the size of the cuts are the same in all visible areas.
The average height of a backsplash is 18 inches, which means on a normal 12-inch tile layout you will have a full tile on the bottom with a 6-inch cut buried into the underside of the upper cabinets. However, with smaller tiles you may find that centering the installation will provide you with the best visual appeal. In addition, the same rule applies to diagonal patterns in regards to height, as you may find certain size tiles leaving you with a sliver cut against the cabinet, with a better visual aesthetic provided by centering everything according to height.
Backplashes without murals can be centered based upon the overall width and height of the backsplash. However, if you have a mural pattern anywhere on your backsplash, you will have to center the backsplash based upon the mural rather than the actual dimensions of the kitchen. The reason for this is that different-size cuts dying into the mural create an instant attraction for the eyes, whereas same-size cuts around the mural give the overall kitchen a sense of centeredness.
If you are dealing with a typical kitchen layout where there is a break in the upper cabinets above the sink with a window in between, you need to center this part of your backsplash around this section. Different sized cuts around the window and into the sides of each section of cabinet throw off the look and feel of the room, so these areas need to be treated like murals in regards to centering.