Is knowing about a criminal past essential?

Recently during a discussion, an associate noted she was upset with her boyfriend for not telling her about a crime he committed in his past. When he was young and dumb, he and his friends decided it would be cool to steal tires off of a car. At the age of 16, he served time after being caught.

Well now, he is in his mid-20s and appears to be one of the most straight arrows you will ever meet. Nevertheless, when she learned about this from his brother, she was upset because he never informed her of this in their three-year dating history. His argument: he didn't think to tell her.

Of course, the room was divided over this one. Some thought that he should have told her and by not telling her, it indicates he could be a liar or there are some other secrets he's keeping from her. Others thought a crime he committed at 16 was no reason to fret, unless it was something involving harming another human being. That, they said, could show potential problems to look for in the relationship.

So do you think a significant other or someone you are dating should inform you of their past? Do you think all arrests or incidents should be acknowledged? What factors come into play? Is the simple act of omission the main problem?