Like, I'm missing four rares, two epics, and one legendary from my ideal deck, what's the best use of my dust?
Do I start crafting from the cheapest? Save up for the most expensive first?
The argument for saving up is, while you are saving, you might open the more common cards.
Well, it depends which cards you're missing. What are they?
Worthwhileness to craft is a mix of:
1. Importance in the desired deck.
2. Reusability in future decks.
3. Cheapness.
For example, for the ones you've mentioned:
Cenarius:
Importance: Medium? He's strong but not the defining card of token druid.
Reusability: Low. Class-specific and not even run in every druid deck.
Cheapness: Legendary.
Keeper:
Importance: High? A key tool vs aggro and the silence is versatile.
Reusability: Medium. Class-specific but played in every druid deck.
Cheapness: Rare.
Contrary to some other opinions, I would absolutely craft key commons. I didn't open even one Harvest Golem for a while and haven't regretted crafting two.