The reason you worry about grain is that all fabrics (even coutil) have stretch on the bias. In order for a corset to retain as true a shape as possible, the bias stretch needs to be minimized.
To that end, I place the bias such that the pull on the panel will be as close to the straight of the grain as possible. That usually involves placing the grain line with equal panel curve to each side, balancing the bias that will inevitably happen towards the sides and ends of the panels. Some parts of the corset can actually benefit from a bit of bias, most notably the bust and the sometimes the hip.
When a corset is on the body, the greatest pressure with be front-to-back, strongest at the waist (where the reduction occurs) and weakest at the very top and bottom. Where you want the pressure to hold, you want the straight of the grain. Where a little stretch is beneficial for comfort, some bias can actually be helpful. Different styles and cuts of corset have different shapes with differing pull and differing stability needs. I suppose in most cases that would be perpendicular to the ground, but I never think about it that way.
For the basic Victorian styles, the grainline should usually be centered on the panels, very close to straight at the waist. For Edwardian styles (particularly swept-back S-curve styles), on some panels the grainline is diagonal to the panel because that is the straight once the corset is assembled. The rippling wrinkles sometimes seen at the hips of Victorian-esque corsets, and sometimes at the bust, are a result of bias pull. When you deliberately pull on the bias of fabric, you will see the same wrinkles as the material stretches. To correct that sort of problem sometimes involves a tilt of grainline so the area of pull (where there is greater pressure in the garment) is more stable. However, sometimes a proper grain for the hip will cause a grain problem in another part of the panel. In such an instance the shape and placement of the wrinkled panel and the adjacent panels must also be altered, or other changes need to be made to stabilize the area. Quilting and cording, or sometimes just the addition of bones down through the affected area can often correct the issue.
When drafting my corset patterns for customers, I always mark the grainlines. When I am making a pattern for my own use, I usually do not mark the grain. For basic Victorian shapes I balance the panel on the coutil, using the ribbing pattern to measure how much of the panel is off-grain.