Salt will repel roots, but only while it is contact with them. Pouring it in the drain lines will only put it in contact with the roots for a couple of minutes. It would have to be in the soil around the sewer to be effective. By the same token, any root destroyer has to remain in contact with the roots to work, and the only time that happens is when the sewer is plugged or about to plug up and then there is not enough time for the RD to do its work.
: What do you recomend for sewer lines that get tree roots in them periodically? I've heard if you pour a medium grade salt into the line from time to time will help keep them away.