I personally SS before every fight and, if we're clearing trash, pre-cast it on the tank so it goes off right as the last mob dies so I don't have to waste time before the next pull. For bosses, if it does a lot of constant AoE damage (e.g. Demon Wall, Titan), I don't bother. Otherwise, I'll apply it on the DPS when I have a healing break or Swiftcast+SS on the tank if there's incoming spike damage (e.g. Tonberry's last Grudge).