Its safe, but in high enough doses will reduce growth or even kill your plants.
I'm currently caring for a lobby fishtank, and the previous caretakers were dosing salt as a disease preventative. As soon as we stopped using salt (keep in mind the people were OVERdosing) we started having bulbs grow that we didn't even know were in the gravel, and our java fern and java moss perked up within days....
It can definately have a negative effect, but I'm pretty sure the salt levels were extreme in this particular tank. They were dosing for 200 gallons when they did a 40 gallon water change on a 125 gallon tank.... idjits.