If you are concerned with the aesthetics of your tiling, you will want the edge tile to be the same on both ends for the area width and the area length when tiling a floor. For tiling a tub wall, you want the edge tiles for the width of the back wall to be of equal size. (continued below calc)
If you know the exact size of your edge tiles, you can start laying the tile from one end of the area you are tiling. If you don't know the edge sizes, you will have to start tiling with the first tile in the centre of the area.
Tiles will expand, if ever so slight, and if placed against a solid, it won't take much for a grout line or tile to crack. Better to be safe and add an expansion gap around the area's perimeter.
If the edge tile becomes too small or large, consider adjusting the size of the grout line or the expansion gap. If nothing works, use a different size or tile for the edging to make it look good.