Is it better to invest in a pivotal dragon's Star Rank?

Raising the Star Rank of a pivotal dragon is one of the most durable investments money can accelerate, because Star Rank is 100% permanent (it never resets between Reigns).

What a star brings:

• + Stats (Legendary +20%, Epic +15% per star)
• Unlocks a Habit slot (roughly one every 2 ranks)
• + Army size led by that dragon
• Sometimes, it switches on a big effect that sleeps until 6★/10★

The right reflex: aim at a pivotal dragon (the one carrying one of your main armies, with a good affinity) rather than the flashiest one. And target the tiers that unlock an effect, not just the number. The same result is reachable for free by farming Relics — paying only brings it closer.