you should only hold hypercharge for wildfire phases if you know 100% well that you will not lose a hypercharge in the fight if you delay it. one such situation is a1s - opener hypercharge, then save it for post jump phases, since the timer lines up neatly.

realize that it's only 5% extra vuln damage that is stacking with wildfire and your other buffs; the turret dmg doesn't affect it. so really, unless you know the script 100%, it's best to just use it on cd to benefit your group more.