In general, you don’t want to restrict the stretch on any of the seams, so I’d place the least stretch going across the center of the cup from the wire to the apex. It is really subjective and dependent on the fabrics themselves. Most of the bra making fabrics have a hair of give in most directions, so its forgiving. If you use cotton woven, you may have to play more with the grains. As with clothing the cross grain, goes around the body, but because the breast is cylindrical, it may work with some fabrics in one direction and not in others. I know this is kind of a non-answer, but the real answer is to experiment and think of how you want it to shape the breast. The top cup might have different rules than the bottom cups, then a powerbar might need to be bias. What I would do is cut the pattern in paper and tape together. Then look at where you want to keep it more stable on the body. A simple manipulation of moving a seam by 1″, could be a completely different grain.