I used to add one teaspoon per five gallons to my planted tanks per LFS. I had some guppies, no other livebearers. Plants grew fine but I realized that with the evaporation going on my tank was probably getting saltier. Sometimes I'd top off no salt. In my new tanks I don't use salt at all.
i was doing an ich salt treatment in my 20g. the cheap pestmart feeder guppies i'd got to be breeders had ich. i did 2tsp per gallon. most of the plants were fine for the whole 5 days, but my crypts started melting, so i started removing the salt after 5 days because i love those crypts (3 original plants have split into about 15 or more), and i didn't really care about $1.50 worth of guppies. still killed the ich and haven't had another outbreak since.