You don't have to experience bad stuff in order to know it exists. A lot depends on the common sense of the child and his ability to appreciate responsible behaviour.
I think a computer should be kept in a 'public' spot in the house. Even Dear Abby would agree with that one.
As seen of the teen's point of view, it can be embarissing if parents see what there surfing about. And if it's there computer, it's there property.
Maybe i was to cynical last post, but it's not only the bad stuff. I just think it's a teen's right to find truth (whatever that is
Don't forget teen's go through some difficult hormone stuff and life questions and still worry about what other people think about him/her.