[Wynott]

I have a bunxh of vertical surfaces in the scene...

Surfaces running along the x axis orient the materials one way, surfaces along y orient the other way.

How is this determined?

It's important for wood grain that they all run vertical.

Do I have any choice but to apply a cloned but rotated material to the offending surfaces? Or is there some other way to control how the material is oriented?

Material is set to meters, not relative.

[kami]

If you put the textures in the material to 'meters', then the material will use the RealScale feature of maxwell.
You can rotate them, if you select the object, go to the object properties and then to the Maxwell Properties of that object. There is a setting called 'RS Rotation' (meaning RealScale rotation, a bit hard to remember because its an abbrevation). There you can rotate the texture.

Also interesting to know when working with RealScale materials is the command: Maxwell_RestoreViewport
which restores the texture preview (for example if you scale or alter an object with a RealScale material)

[Wynott]

Perfect. Thanks kami.

[JDHill]

The way of handling it natively in Rhino is by using:

• a) For an individual surface, Dir > SwapUV.
  b) Object Properties > Texture Mapping.