I run it most of the time in my mixed reef. I have quite a few leathers, and I have witnessed them bothering LPS that are near them. I use about a cup per 100 gallons. It's probably not enough, but I do change it most weeks.
I agree with C-Man. It's a really good idea to have it on hand. If you have any kind of disaster in the tank, you'll want to have some available to use.