With very few exceptions, I never use the mana node when spending emblems. It doesn’t make a difference on most heroes unless they are combined with other mana speed buffs.
The only exception I made was Lancelot from Avalon. But that’s because his +24% mana boost combined with the mana node makes him a fast speed hero after he uses his skill once. Again, these are very specific cases and typically only useful when combined with other mana boosting heros like Friar Tuck or Ariel.
For the maths on that, this topic lists the mana speeds you need to shave off 1 shield from a match to charge hero speeds, and also lists the %s that each mana troop boosts speed by: Optimal Mana Bonus Numbers for Every Mana Speed (Now including Ninja Heroes) - #3 by thing2