The insertion of balsa or carbon rods for long spans tries to fight buckling and vibration.
Any bent rod is more prone to suffer buckling, just avoid bends and run rods as straight as possible.

Good reading about buckling:

http://en.wikipedia.org/wiki/Buckling

The difference in diameter is important (25%):

0.074" (1.80 mm) for a 2-56 threaded rod

0.095" (2.42 mm) for a 4-40 threaded rod

(Edited to correct mistake / See posts #6 and #8 below)