Most of the totally rotted out springs I have seen are the ones that were greased and wrapped in burlap and tin. Trapped moisture inside the spring pack and accelerated rusting.

Some years had no rubber in the shackle kits (AD trucks for instance) others had rubber bushings (my '35 has rubber)

Any way you can lube the springs without introducing something that will attract dirt or hold moisture is fine. Pretty much limited to the poly or teflon strips or dry lubes per Ralphy.