I have always been one for the "benefit of the doubt" philosophy. Not because I think people can change, but the issue of cheating is almost always situational. If someone is in a mindset that they can't be monogamous, they will cheat on every one of their girlfriends. However, I have been the girl that a serial cheater did not cheat on. I have also been the girl that a "never-before" cheater has cheated on. If something is wrong in the relationship, the risk of cheating is always present (which, by the way, is NOT an excuse).
For me, trust issues are important without even bringing past infidelity into it. If I was dating someone had cheated on his ex, I would be a bit wary. There would be that underlying paranoia, I admit. But I usually have gut feelings, and if it's worth it, I would go for it. Everyone has a past, and everyone makes mistakes.