Welcome to the Gauge!
For me, using salt as ballast is definitely NOT a good idea. With changes in humidity and temperature lots of salt grains will disintegrate into a very fine dust which in turn will be blown all over the place. Also into your engines...
Together with air humidity this powder will wreak havoc with ferrous metals (steel axles), soldered connections (e.g. in electric motors, in turnouts, at feederwires soldered to the rails...)
I'd say: Hands off from using salt as ballast!
Ron