You want your GH above 5 or 6.
Whether pH swings has more to do with whether you are doing good maintenance than anything else. If you are doing weekly water changes of 30 - 40%, it's unlikely that your pH will swing unless you have a very high fish load. However if you tend to go for a month without water changes, yes in that amount of time the pH can crash. I've had this happen, but never lost fish over it. Adding KH has it's risks too, because a slight mismeasuring from one water change to the next will cause a KH/pH swing. KH fluctuation is much more harmful to fish for the most part than pH fluctuation. So I don't know if I would jump on that bandwagon, if I were you. Once you're on, it's not easy to get off.
However, on the other hand some fish require a higher KH to thrive, so in that case maybe you should if you are keeping those types of fish. Personally, I choose fish that suit my water type. Tetras and angelfish do great in soft water.
What you should do, though, is make sure to drip acclimate your new fish. This will prevent them from going into shock if they are coming from harder water to your softer water.