Unfortuantely, there is no right or wrong answer to this question. That's because some people are made better by fiber and some are made worse. You'll never know until you try it.
Insoluable fiber from real food, like broccoli, bell peppers, etc. tend to bother most people, regardless of their type of IBS; tends to give them gas, indigestion, bloating, etc. Raw veggies are to be avoided because they're hardest of all to digest; most people, though, can handle them if they're cooked. Some people can tolerate al dente, while some can't eat anything unless it's cooked to the consistency of a limp noodle.
Of the fibers you get from supplements, if you have diarrhea, you will want a supplement that has more insoluable than soluable fiber; this helps builk stools. The opposite is true if you have constipation: more soluable will help moisten your stools and make them easier to pass. Supplements usually give everyone some gas to start with (starting with a smaller than recommended dose and working up to the recommended dose can help avoid a lot of this problem), but most people adjust and get over it in a week or two.
Personally, I have to be careful with fiber. I can eat veggies al dente no problem. I can't eat most fruits, though, at least not raw. I can't take supplements, though; if I'm have C, they make it worse, and if I'm having D, they make it worse. I also got bad D after eating blue corn chips; too much fiber per serving. So no fiber cereal or anything like that for me.