Originally Posted by
Kurenai
It's just a matter of preference. I already full-cleared every dungeon by the time Centy was introduced, so I'd think of it as just "cherry on top."
Given their recent introduction and the length of time it will even take to refine a Centy using Petite Centies though, saving or using them on your waifu or favorite should be the soundest choice, then moving to the ones you know you will use for a long time (if you will have enough spares to even bother with, that is...)
Keep in mind that only max-level bloomed rainbows can make use of the Centies. For 5★ or lower, they need to undergo rarity promotion to 6★ (as long as they're eligible) and then maxed to Level 80 as well.