BLM

[Foullacy]

I cast Thunder III cause it looks better.
I'm also fairly certain things like trying to hold out a thundercloud proc to reduce dot clipping or maximizing your firestarter procs by interrupting your fire 1 casts (or especially blizzard III when going into regen) makes a much bigger dps difference than this.

The tooltip on the thundercloud buff implies square probably meant for thundercloud to be determined by what was hard casting. If it's not that in-game, they'll probably fix it.
Thunder 2 hardcast + Thunder 3 thundercloud proc might add something interesting if the proc wasn't so damned low. I think this whole debate is a bit overstated, even for serious min-maxers.

If you read the tooltips for Thunder I - III you would see that the potency increases with each rank that is hard cast to trigger Thundercloud.

That being said, it is still optimal to use Thunder I because it syncs up with the final tick of regen in Umbral Ice allowing you to maximize your casts of Fire. If you're casting Thunder II or III during regen phase you're clipping regen and wasting time you could be casting Fire instead.

Now if you went into regen phase with Firestarter, you would probably see a DPS increase using Thunder II over Thunder I since you have to hold off on Fire III for a few seconds to get the final tick of regen. Consuming Firestarter before regen phase is ideal because if you hard cast Fire III with 3 stacks of Umbral Ice you can cheat a hastened Fire.