Is it ok to date a friends ex?

I've heard before that it's basically for forbidden to ever date a friends ex. Even moreso if you're really close. However, I want to know what you guys think of this. Do you think it's ever acceptable for someone to date their friends ex? Are there any specific rules you'd consider? Example, if your friend was the one who broke up with her for his reasons. Personally, I don't think there should be an issue if enough time has passed and the circumstances for the break up weren't awful. For instance, if my friend broke up with a girl because he wanted his freedom, there shouldn't be an issue with another friend pursuing a relationship with her. However, if my friend broke up with a girl because she cheated on him and really hurt him, then I don't think a relationship should be pursued. Your thoughts? I'd put a poll in but polls are dumd