When Not To Use Salt
Live plants - If you have a tank with live plants, avoid using salt. Plants can be damaged with a relatively low dosage of salt, which is one reason its best to treat sick fish in a hospital tank rather than your regular tank.
Scaleless fish – Scaleless fish, particularly cordydoras, are very sensitive to salt. Even a small amount could harm them. Tetras are also fairly sensitive to salt. Contrary to popular view, it is not advisable to add salt to your aquarium on an ongoing basis unless the fish require brackish water conditions.
from a website. two reasons for me NOT to use it, and no definite ones for me to.