While that is indeed a simple solution, it can quickly become tedious.
Consider the following workflow: SketchUp to Blender to Unity. SketchUp is great for modeling buildings/interiors quickly, but lacks a UV mapping feature. Each time I export a model to Blender, I have to find the floors and rotate them. On a multi-level building that requires constant adjustments based on user testing, it turns into a lot of work, for something that really should be a feature of the texture. I'd rather spend my time working on gameplay/content.
Strangely, it seems that some substances do feature a rotation parameter in the inspector. Italian Fieldstone is one that comes to mind. It seems that some substances just don't have that parmeter (e.g. Wood Cherry Floor).
@Jeremie - can we make this a thing? I think that an update to some of the parquet substances to include board/grain orientation would be very useful to your user base. Am I wrong?