Should I always fill my troops to the maximum in Dragonfire?

Yes, always. A dragon's troop volume is its fuel and its punch: it's the base that all your multipliers (research, affinity, auras, triangle) apply to.

Concretely, each dragon has a troop capacity (≈5,000-6,090). Three full dragons ≈ 15,000 troops. That's also your durability: a dragon emptied of troops flees the fight.

The attrition math shows it: against a full defender, an army loses ~300-600 troops per round. Set out with half-filled dragons and you retreat sooner and land fewer useful waves.

But mind the reverse trap: you fill troops to the max, you never choose a dragon for its troop capacity. Capacity is a routine setting, not a selection criterion. You pick dragons for their Orders/auras/affinity, then you fill.