I have alternated between using Round Robin thunderstorms and popping 4 at once since the spell was first introduced. Honestly, I really like the 4 at once approach. The only exception is places where I may want to be able to bounce a few different people off a cliff.

Maybe it's just my imagination, but when I pop all at once, the small delays between the casts going off seem to push people much farther away at times.

Anyhow, I don't think there's a "wrong" way to use it, but if the damage is significant, I think there are good arguments for using all at once. Fire Nova+Thunderstorm is a pretty nice combo when a couple melee swarm you.