I want to look at this a bit differently for a minute, because several people are saying “more HP = living longer = good”. Conceptually, yes, I agree. But realistically, I’m not sure it holds up.
At the OP’s current level (71), his Gunnar is level 3 in Green, and will not be level 4 until he hits 84. Gunnar’s base HP at this level is 438,703. Lacking information on the gear on Astrid, we’ll just work with the numbers provided. Also assuming that the OP has the relevant HP research through Orange (17%), this Gunnar should have a 23% boost to HP as it stands. So Gunnar has 539,605 HP with this Astrid (no other runes, and no gear). If the OP takes the balanced approach being asked about, Gunnar will get another 5% HP, bringing him up to 561,540 HP.
At level 73, a player can have a level 27 Dark Flak tower, which does 259,533 damage with a normal shot, and 537,770 damage on supershot.
So what do we make of this information? Well, I’m not inclined to say that increasing HP really does anything to improve the survivability of this dragon given these parameters. As it stands, the dragon can withstand a single DF supershot by just a tiny bit; an additional 5% HP means that the dragon would withstand that single DF supershot by just a tiny bit more. Given the nature of incoming damage (i.e., it’s not just one tower doing the shooting), any amount of residual damage is going to clean Gunnar up if he gets hit at all. The HP increase is not enough to conclude that “more HP = more survivability”.
The way that HP gains work in the game is something I’ve mulled over for a while, specifically concerning the in-game advertisement that “warrior dragons have a ton of health, and are good for brawling your way through a tough base” or whatever that fortune cookie statement was. Yes, warriors have more health, but given the amount of damage that towers do, I’m not at all sure that the boost to health is more than superficial. Hunters, for example, often just avoid the damage, which is better than soaking it anyway.
Now - because spell damage is directly tied to HP, that is a good reason to maximize the HP on the dragon. I’m not entirely sure whether Gunnar’s spell kit is HP based or not (I’ve forgotten; we tested that thing last Fall). So I do advocate stacking HP - but only because of the increase to damage output that comes with it.