Yes to the Titanic question. If you use screws with low stiffness, you can't hold your work piece in place with high stiffness so all the mass / stiffness due to large rails, big structure is wasted. If one part of the machine is much more flexible than the rest, that part will end up dominating the stiffness of the entire machine. I'm working through this on my milling machine design at the moment. THK bearings were too flexible to achieve the stiffness targets I wanted so now I'm looking at hydrostatic bearings, just by way of example.