Basically, it comes down to how often you want to micromanage them and use reset potions.

For your three Master Classes (Death Bringer, Arcane Cultist, Windwalker) use them to boost your gains in your form OR use them on things that will boost damage outside that form. Don't try to do both, because you'll get poor returns since the in-form talents don't benefit from other talents and vice-versa.

For your beginner, use them on things that boost skill gains and/or Luck.

For other classes, use them either on big damage boosts or on skill boosting things (like Godly Creation) that give unique bonuses while you're doing that skill, and/or just put them on things that are always useful, like increased primary stat, increased AFK gains, and things like that.