It really depends on the circumstances. Normally, I don't even consider trolling. However,if someone, or multiple people, in a forum are being total dicks about a topic, I just go for broke. Case in point: a recent topic came up on youtube about Ryan Dunn's death.
So, I congratulated him on his Darwin Award. The next thing I knew, some guy had threated to force me to watch as he burned my family alive. The same guy was also using rather poor grammar in incinuating that he thought I was a homosexual, but that's to be expected of the internet. At that point, I just decided to have fun at this guy's expense. By the time I was through, I had the guy admitting that he could understand why I thought that HE was gay.
Trolling isn't a card I play very often, but I'm certainly good at it, if provoked.