It is a known fact that sodium can affect the plant's ability to take in nutrients. The degree to which this affects the plants varies a lot depending on a lot of things, like how much sodium is actually in the water and what type of plants you have. But it is generally agreed that it is never a good thing. That's why many people with planted tanks use unsoftened water or use potassium chloride in their softener instead of sodium.
Fish on the other hand have widely varying preferences for sodium, some will do better with a little sodium and others will do worse. But for plants it's pretty much all bad, although you can often get away with it without major effects. Tetras are one example of a fish that it is said does not really appreciate sodium in the water, although I get away with it with mine. I also get away with it with my plants, but I'm having some unexplainable troubles lately so I'm going to scrap the salt and see if things improve.