Grainline placement is relative to both the design and the fabric. I know this isn’t the exact answer you are looking for, but when considering the placement of the grain you want to make sure not to place the non-stretch position of the fabric on any seam. This can be challenging since the seams can go all over the place. Look at the cup and think of how you want it to fit. You want stretch to go around the breast and not cause flat spots or dimpled seams. There is no one right answer on where to place it. I would make a paper cup and look at it 3-dimensionally and determine where the most appropriate places to have stretch are and where to restrict it.