Well no, sometimes maybe.
Use it at the opener as usual. Save the other DS for when you have some sweet procs/buffs and such. Or when the add/boss takes more damage for some encounter specific reason.
It's quite nice to have when you and your guild really needs that extra damage too.
Just popping it without any procs or reasons seems silly in most if not all cases.
best way is to use it just like you would before, you just get to use it more often. almost 4 embers, pop and cast for destro ...trinkets procced? trigger and refresh dots as aff. etc.
It's impossible to stack the two DS, no matter how much you press it, you can only use the second charge after the first DS runs out.
Taking that into account and the fact that Imp Swarm's cooldown is variable, what I usually do is use one charge of DS in the opener along with Imp Swarm and the Synapse Springs engineering CD. Imp swarm's CD being about 1min30s, and Springs being 1min, I can make sure I can always stack all 3 CDs every 1min30ish instead of every 2min without the talent.
All in all, most in-raid situations will require either Archimonde's Darkness or Kil'Jaiden's Cunning. I see Mannoroth's Fury as a more trash-oriented talent.
Aha. I thought I read off one of the spec threads that the buff does stack. Thanks, guys. It's no a no brain-er at this point. I here, for Demo at least, Mannoroth's Fury on the right fight out shines Archimonde's Darkness by a fair amount.
Unless there is a mechanic that lets you do additional damage to the boss during a certain phase, save the second charge of dark soul + your intellect pot for execute phase.
Definitely save a charge for when you have multiple procs up at the same time. Also make sure to have a charge ready for any execute phases, or increased boss damage phases.