Don't know about the rock salt, I would be concerned without some further confirmation.
I have read that in order to reduce the risk of fungus causing damage to a lens surface it pays to expose the glass to sunlight every now and then. Of course you
do not look through the telescope at the sun without proper filtration. The UV rays will kill any fungus spores that can lead to surface etching especially if the scope is stored long term and there is any moisture present.
I read it in this book which I found in the local library;
http://www.amazon.com/Care-Astronomi...7046966&sr=1-1