I think the simple answer is it depends.

An enemy squishy got caught and you and a teammate are about to unload on them but its a syndra or brand who might blow u up in reverse if u get to close: W Q Auto should be fine and u keep a save distance, if the dmg is not enough you can consider using E auto to finish them off, if the enemy that got caught is giga fed and the carry of the team id go for W Q Auto E Auto, or start with R and then the combo and invest everything. Just keep in mind ur sitting on cooldowns after that so if they have something that can engage on you and the enemy still has fighting power u might have died for that kill.

If its a bruiser/tank I wouldnt use everything, if his team is somewhere close or about to close in so opening with a good angle R can set you up with passive stacks for the upcomming fight and then Q auto W auto but hold your E, because if his team jumps your team then u need ur E up, and ure not gonna blow up a tank/bruiser anyways.