Like I did in RL with my own three kids, I try to bring out the best in each of my Simmies. So, yes, I do attempt to get their values in. With the Toddlers, they just seem to level up on their skills without much help. But, you do have to work at teaching them to say sorry and learn to say please and thank you.
I've yet to play a Sim who is downright evil. I used to give each Sim in my household one negative trait. It was far easier when we had five slots in Sims3. For instance, my Sim is a Genius, but he's also a Goofball. In Sims three this same Sim had the added attraction of being clumsy. Yet, he rose to the top of his chosen career. He became a World Renown Surgeon. (And I can hear you asking, how can a clumsy Sim become a surgeon, let alone a World Renown one?) LOL That's the Sims for you. Oh, and he was also Childish.