Burning Deathflare or painflare isn't a bad choice.

If it hits more targets it is a dps gain, simple as that.

Each GCD of trance using Ruin III is worth 44 potency (additional). Each GCD of blizz II with trance is worth 5*target (additional). Hitting an additional target with Deathflare is worth 440 potency (additional).

Losing even 1 target of deathflare is worth more than 10 GCDs of Ruin III. You'd have to be AoEing something like 90 mobs for the loss of one mob to be worth using Blizz II instead XD

I pop it as soon as any target hits 10% hp, unless they're big bulky targets that you know will take a bit to die.