As a general rule you want to center the installations as often as possible for the best visual result. Any cuts should be kept to a minimum, and if possible you should lay out your tile installation for the tub so that any cuts are at least half of a tile, or close to it. If you have a vertical termination point this should be wherever the full tile level terminates. The industry standard is 7 feet or wherever full tile ends near that mark.
While centering a tile installation for your tub is generally the best option, it does have some downsides, such as determining exactly what is center. When you are dealing with one or two outside corners for the edge of the tub enclosure as well as three to four inside corners a multitude of cuts will be required. In this case, centering the installation over a certain point may not lead to the best result depending on the cuts in other sections of the enclosure.
When you are laying out a shower and tub enclosure there is one thing that is of primary importance. The most eye-catching areas are the spots you want to choose for full tile and/or large cuts because these are the areas the eye is attracted to when you walk into the bathroom. Hidden areas, such as an inside corner where the shower meets the tub and that is hidden from view unless you are in the shower, can be made up of smaller cuts of tile.
You have to look beyond individual cuts when planning the layout for a shower and tub enclosure. Lay out your tiles on the floor to get an idea of the tile-to-tile spacing and then use those measurements to check the size of the cuts into your corners based upon various starting points. The most visible point in most shower/tub enclosures is the face of the tub and the wainscot above it where it leads into the shower, with everything else following off the most visible points.