I'm with Red and Matrix on this one and am very strict about it.
Cleo's argument is a real good one and made me stop and think last time I read it - and again this time - but I also read a lot of stuff about this subject recently and I notice that particular attention is paid by the source sites (one was a US educational establishment and one a UK government site) to pages which contain both a warning AND hardcore content; the figures I saw quoted 72% of "warning pages" had adult content on them, (which is obviously drawing more attention to the phenomenon).
I just want to steer clear of the whole shebang and make sure I link to pages which are not attracting unwanted attention for any reason so I don't allow it at all.
(Just out of interest - I also noticed that the word "warning" is not the only thing which qualifies a page as a warning page in this respect).
I also think it's silly to risk losing listings or liberty so I always build with "censored" entry pages on my own free sites
