Yeah, I try to always start fights (IE: If I'm running to someone or they are running to me) in Firemode so I can FB and switch to RoI and have an interrupt. Unless its UB. I dont like UB running up on me without DF to interrupt or extend the cooldown of Spit.
But if I'm supporting, I always go Firemode first, unload the Fireball, Switch to Frost, RoI/BotS, switch back to fire and Fireball.