As a Roblox developer, it is currently too hard to orientate materials, especially when using non-block Parts. Material orientation can drastically affect the appearance of a model, as WoodReviewer would be all to happy to argue, I’m sure. 
At the moment, to orientate materials without plugins for block Parts, you must rotate the part and copy-and-paste Size numbers into different axes. For this, there is a plugin: MaterialFlip by stravant, which automates the process; however, this does not account for non-Block parts, as they are assymetrical on at least one axis. This means certain Parts are “locked” onto having materials orientated in a certain direction.
The workaround I am using when dealing with non-Block parts is to create a NegativePart away from the Part whose material I want to orientate. Before negation, the prospective NegativePart is orientated as a Block. By selecting the NegativePart first, any other Part unioned with it adopts its material orientation regardless of shape. This is not favourable but necessary to preserve orientation.
If a way to edit the orientation of each face of a Part was possible (or just each axis if the former isn’t viable), it would help a ton!
Thanks for reading!