I use stress coat constantly. Whenever I am adding a fish to the tank I add some to the fishes bag AND to the tank itself. It's also good to use after a water change, or a change in aquascape, or a bout with ich or another disease, etc.
Basically, any time you feel your fish are stressed out it's good to use. It replenishes the slime coat of the fish which is affected by them being stressed out.