I've browsed around the forum, and people seem to suggest a .2mm tolerance for stainless steel fitting pieces, but I am concerned because shapeways states that the accuracy for stainless steel is only within 2mm of the design. With that kind of accuracy, I am concerned that the two pieces will not fit together in the end. Does shapeway actually care whether or not my pieces will fit, or is it just "luck of the draw"?
The variance will depend on the design of the product, wall thickness and form. Shapeways does of course care about the quality but there is some warping and variation in the stainless steel process, less so in other materials.
I would order in White strong and flexible at the same time so you can test against each other