I usually always play a good guy, mostly because my own policies go strictly against most "bad karma" choices, and I don't want to encourage parts of myself that I don't agree with in general.
However, in say, Mass Effect, being renegade isn't actually particularly bad at all. You still save people, you still get stuff done, you just do it differently with a different mindset, so I didn't really mind being "bad" in ME 1&2.
In games like Fallout 3 however, where the bad choices truly affect a big or significant amount of people, then... no. I can't, spoilers, blow up Megaton and live with a fresh mindset in Tenpenny tower, wiping out an entire city isn't my idea of a good choice in any way.