Actually, even if it change nothing at 100% affection, it's still better to evolve a girl with 100% affection than with 0%. When you evolve a girl, she is back to lvl 1 and get lower stats than her lvl50/60 pre-evolved form. But if she is 100% affection when she evolves, the double affection bonus compensate the stats loss and she will have almost the same stats than her lvlmax non evolved form. It really matters u til you get enough good evolved girls to make solid squads.