I give my sims all the good traits they need to succeed at whatever I have in mind for them. But that doesn't always take five traits, especially for some self-employment jobs. So I fill out their traits with whatever will fit the story I have in my head. Sometimes I use traits that are normally considered negatives, like Snob or Neurotic, but that create a more interesting personality or fit with a sim's overall arc.
I rarely use outright "bad" traits (Coward, Loser, that kind of thing), unless I'm trying to make a point, and then only for sims I'm done playing with or random townies who'll never be active. But it is amusing to watch those sims behave badly from a distance, as long as I don't have to deal with the consequences myself.
