Shafting would, or at least SHOULD be heat treated to a springy sort of condition already. So I'd consider that a wise step for use in a jig of this sort.
I'd even go so far as to suggest you modify the jig to allow the use of different sizes of rod and buy something larger for the longer span projects. Perhaps something like 5/8 thick wall carbon tubing? Even if you had to combine two different sizes it would be a step up in the stiffness and support department. For example a nice straight piece of 3/4 carbon or Cr-Mo tube for just ahead of the main spar and a 1/2, 9/16 or 5/8 tube mid way between the spar and TE would be that little bit better than two pieces of the 9/16 by themselves.
Also consider that the stiffness of a rod or tube goes up by the fourth power of the diameter. So while the difference between 1/2 and 9/16's seems like very little it actually produces a very noticeable increase in stiffness. And the less deflection you have per lb of force the easier it is to keep the wing true during the build.
But of course using such large size rods or tubes will only work on thicker airfoils that have room for the holes in the ribs. But then thicker airfoils of that sort are typically the types that are best built with such a jig anyway. The thinner airfoils more often lend themselves to building on a flat board with simple packing where required.
All this is why I've never actually bothered with a wing jig that uses rods for rib alignment. The concept is a nice one. But I'm a stickler when it comes to details like unavoidable flexing. Instead I prefer to make up a "bed" that rests on the building board or bench for the whole span. Or in other cases I've used airfoils which I can build on a tilt with the trailing edge flat on the board and a support strip located up by the lower spar notch.