Another thing to keep in mind is not just the hero but how close your mana troops are to the breakpoint and whether they’d benefit from the talent or not.
Using Kerb's 4* Troop leveling chart, hurray! as the marker, the known breakpoints for mana troops are lvl 17 (slow goes from 12 to 11 tiles) and 23 (average goes from 10 to 9) (Number of Tiles (Regular vs Ghost Tiles) Required to Activate Mana (based on Mana Speed)).
According to the first mentioned chart, this is 11% for slow and 13% for average. Fast heroes gain no realistic benefit until lvl 29 mana troops and that’s still not a practical benefit (8 tiles to 7).
So for instance, the mana talents on a Hansel cleric or Caedmon druid are meaningless, so it makes no sense to pursue those benefits. But for an average mana Rigard or Melendor, it drops the troop requirement for a tile benefit from 23 to 17 (for Rigard who only gets +2%) and 23 to 11 for Melendor since he gets +4% at +20 talents. A +19 Vivica also would gain a benefit at a lvl 11 mana troop as well (12 to 11 tiles).
Much more math intensive, but some heroes benefit differently.
Edit: some classes also get mana benefits sooner up the tree, Rangers get one at +8, so someone like Red Hood might be able to take advantage sooner than another class.
Doing that math did change my mind on whether or not to do +19/+20 on my Rigard, Wilbur, and Sabina since those classes all benefit from having lower level mana troop requirements. I wasn’t planning on going beyond +18, but my purple mana troop is easily within working range or beyond, and my red is past it since monks get +4%. Hmm…