Should I always deploy all my armies in Dragonfire?

Yes — keep your 5 slots always full once you've unlocked them. An undeployed army is power doing nothing.

But "full" doesn't mean "identical". Prepare a fixed, varied set (armies are locked once launched; you swap at the castle):

• 2-3 "long/combo" armies: a Center with a strong aura, a single damage type, stacking debuffs — built to last the 10 rounds and grind down big targets.
• 1-2 "burst/mini" armies: a Center that hits hard raw, to clear small objectives fast.

That split covers every case (big target vs. small tile) without recomposing every time. And since a full target needs several waves, having 5 armies ready means you chain assaults instead of stalemating.