I add approx. 1-2 Tbs of Api salt every other pwc or when I notice my Mollies are acting sluggish. I use it primarily to help w/ their gill function and it works well to get them out of their "funk". Some ppl swear by it and others think it isn't necessary, (especially if your tank includes freshwater fish). My tank has a variety of fish, including a Clown Loach that could get "burned" by too much salt. This is why I only add a very small dosage and I have yet to see any negative effects to any of my fish.
I add the salt in the following manner, (although others may do it differently)... when I'm doing a standard pwc, I remove approximately 25% of the tank water. Once that's done, I add the salt, (along w/ the appropriate amount of Prime), in the first bucket of water for refilling my tank. I fill the bucket w/ warm water and stir it around enough to help it dissolve some of the salt, (not all of the salt will dissolve btw). Before refilling the tank w/ the treated water, I allow it to cool down to the same temp as the water in the tank.
When filling the tank back up w/ the first bucket, I leave approx. 1/4 of the water in the bucket and top it off w/ more water and repeat this for every bucket. Eventually the salt will break down enough by the time you pour in your last bucket.
Honestly though, if no brackish fish reside in your tank, it isn't necessary to add salt. Not to mention if you add too much it could harm your fish. My tank includes Mollies that do best w/ some salinity in the water. Along w/ them, I have a variety of other fish that could become sensitive to high salinity levels. This is why I use a much smaller amount than what's directed so I don't chance harming any of the fish.