The question is pretty self-explanatory.
Personally, I usually avoid using my TS2 game to make my characters for original writings, but sometimes there comes a sim that's born that ends up having a fascinating life or way to do things. Those sometimes get their own vignettes or sketch stories. Appearance-wise, sometimes I do mess around with Bodyshop to see how what I imagine translates visually, but never with important main characters.

I've done that once... with what I consider the worst, stupidest thing I've ever written, something I'm still trying to forget, six years later.

Other than that, though, no, I never use Sims to create my characters. That's mostly because I'm utter shit at creating unique-looking Sims. It's a good idea, though, and if I had any talent at making Sims, I'd totally do it for all my characters.