Yes Rock Salt does work good for killing Tree Roots in a sewer line.
The Rock Salt Kills the Tree Roots in your sewer line by drying the tree roots out.
Pour about a cup of Rock Salt Into the bowl of your toilet and flush the toilet. But first I would also pour a cup of rock salt down your sewer pipe through the clean out before flushing some down the toilet.
This will get more rock salt down your sewer pipe and too the roots.
After putting rock salt down your sewer line and flushing rock salt down your sewer line don't use the sewer line for at least 8 hours so that the rock salt can stay in the sewer line and do it's job at killing the tree roots.