All i have to say is, it depends on your playstyle, and the heroes and troops you have.
For me, i fully emblemed my Mitsuko for her to have the 4% mana bonus so that I can support her with a level 11 mana troop. Doing so, she fills mana in 9 tiles or 3 simple tile matches of red tiles. While my level 25 mana troop will be able to support my Wilbur so that they fire at the same time and my level 6 mana troop supporting my Boldtusk with maxed costume bonus. My immediate plan is for my level 6 mana troop to get to level 11 so it will support my fully emblemed Wilbur and have my level 25 mana troop support my Falcon. I still have a level 2 mana troop to support my BT+19 and all of them will fire in 9 tiles.
I also fully emblemed my Telluria way back in March since that 20th node of defense bonus will help her get a bit more sturdier as a tank, especially when the paladin talent kicks in.
I also have Kageburado and Gravemaker both at +19 and have almost 400 barbarian emblems but instead of placing those 250 emblems on any of them, i rather use them on my Ametrine I am currently leveling.
For mana bonus of 4% (the rest of the classes only provide 2%), only monk and druid classes provide that much bonus on the 20th node. For paladin class heroes serving as tank, that 20th node is a must for such hero to be more effective and efficient on such role. For clerics, i dont think there is much need for the 19th and 20th nodes especially more so on offense.
Again, there is no tailorcut or ideal answer to the OPs question since each of us have different heroes and troops.