Whenever I upload a model in Y-up orientation (using maya) the render shown on shapeways is always rotated showing the underside of the model, suggesting that shapeways uses z-up. However the dimensions shapeways lists is correct (6cm high, 16cm deep).
When I rotate the model in maya to be z-up then the render looks normal, but the dimensions are wrong (16cm high, 6cm deep).
As my model is in multiple separate pieces, I need them to be level so its not trying to print parts in mid air floating over one another. Which way should I orient the model?
If its just "I need them to be level so its not trying to print parts in mid air floating over one another" that's an issue, no need to worry, as layers are printed, a support material is used to support the next build layer. The Shapeways video White, Strong & Flexible 3D Printing shows what happens.
Otherwise, orientation is personal choice. My software uses Z up, I've got into the habit of laying the model over with Y up before upload.