In general the unfortunate thing about the Mana Bonus is that unlike all the other stat increases or Healing/Critical bonus is that you can make it redundant if you do later decide to increase your Mana Troop to the point it would have lost the tile anyway (it’s a shame it isn’t a permanent -1 tile boost on top of any troops) and then mean you’ve missed out on the benefits of the other branch.
There is also an opportunity cost to consider that if you only have one Mana Troop in the colour sufficiently leveled to make that +2% work are you happy committing the mana troop to that one hero for the long term to get that benefit.
Generally speaking I prefer to try and keep my options open in the long term more rather than make that commitment so I’ve taken the approach that I only go for the Mana Bonus if the hero is Average mana, if it isn’t Average I won’t go for it.
Why average? Because I’ve got Ariel and Lancelot. The +24% Mana Buff from their special will shave 2 tiles off any Average hero with even the +2% Mana Bonus so even if I use Critical troops on the Average hero they can temporarily become fast heroes when buffed by one of those heroes. So essentially if you have a +24% Mana Buff hero like Ariel, Lancelot, Khagan, Boss Wolf etc you might use with them then the +2% on an Average hero allows you to give another hero in your stack the Mana Troop and shave two tiles from your hero whenever a +24% mana buff is in effect.
Otherwise if they’re not Average the mana bonus doesn’t get me an additional benefit I can’t get from powering up the troop further (and means missing other benefits if at a split in the talent tree) and I believe there were hints that with the new building upgrades coming soon there may be the possibility of getting troops from upgraded Training Camps so levelling troops may become faster in the future.
Though naturally this is just my opinion and approach, there can easily be made a case for the likes of Fast heroes that no longer needing to take troops up to Level 29 means you can start on a second troop soon etc so ultimately this one may come down to people’s own preferences/playstyles.