I'm normally a nice guy (one of my colleagues calls me the nicest person on the IE team.) I actually take this as a complement.
However, sometimes it's useful to be the bad guy. I think it's worthwhile to be able to have a wide range of skills to use in negotiations. This is something I coach my team on; don't always rely on one or two tactics. Understand the situation you're in and pick from a range of choices.
That said, some tactics will simply not work for some people. I think I hit one of my limits today. I was upset over a stupid cross team interaction today and tried to be the surly bad cop. It was not successful. In fact, it was kind of gross, like a metrosexual trying to hit on a woman by acting like a jock. The same action would have been fine from someone else, but it looked dumb coming from me.
Know thyself I guess.