Migrants to IJ, 2007

I don't know for sure, but my interpretation of the FAQ is as follows:

"If you frequently post material to your journal that can be considered of an "adult" nature, InsaneJournal asks that you place a warning in your User Information ("bio") to that nature, warning that underage users should not view your journal. This is not mandatory, but can serve to protect both you and InsaneJournal."
---Not necessary to f'lock indecent material in your own journal. Warning of adult content on userinfo page advised but not mandatory.

"If you are the maintainer of a community in which users regularly post explicit images, you must monitor the membership of your community carefully to make sure that all of your members are of legal age to view such material."
--- This one's kind of ambiguous. I'm assuming this means that owners of adult communities must make sure members place age statements on their profile. Now if adult posts are not f'locked, this age monitoring is pointless since anyone can read even without being a member.

So my interpretation is that on adult communities then yes it's necessary to f'lock adult material, but on a personal journal it isn't. If the actual truth is different from that, then the FAQ needs re-writing.