if you want to make the best of your card, always max it out before enhancing. For example...if you have a bunch of princess's. max 2 princess out then evolve them. if you max out first, you keep 10% instead of 5%. THEN.....max out princess knight (evolved princess) and if you REALLY want to make the best of her.......max out 2 more princess's. Evolve the 2 princess's to make a princess knight (Evolved princess) THEN...max out THAT princess knight and THEN evolve it with your OTHER princess knight. And you then repeat the process for the next two evolves.
As far as stats.....off and def. stats don't matter. but OVERALL stats and rarity DO matter. You will save money if you evolve normal cards into high normal cards before evolving. You can max their stats or w/e but that would take forever and i'm not sure if it would be worth it.