When you mark out the furrows for a field OptiSurfaced with a 1D or 2D design, ideally you should use RTK tractor guidance on the same row direction (with A point and direction setting) as the OptiSurface design main slope bearing. This is more crucial for an OptiSurface design than for a planar/laser design because OptiSurface generally allows more variable cross slope. If you create a different furrow direction then you may create areas of the field (where there is high cross slope and low mainslope) where you create reverse grades and ponding.
Below is an area on a OptiSurface 2D design that has high cross slope and low main slope which illustrates this scenario. As you can see you need to be offline a lot to create reverse grades but as you move off, the mainslope will be reducing.
Note: This does not really apply to OptiSurface 4D designs because these are only for fields without any furrows or beds (ie growing on a flat surface).