Feed the lowest manyus to things that are below level 30, feed 3* manys to things that are below 50. Feed 5* manyus to things that are above evolved level 40ish.
As for what to do if you don't have any lower than 30 girls. I tend to save them for girls that will evolve soon so they get that first boost to 20 after evolving. If nothing like that i just feed them to something else as i keep one copy of each girl no matter what rarity. There's always something to waste them on.
Otherwise i guess selling is what some do.
To answer the topic: The gold dailies are almost never worth it.