I have my journal set as 'viewer discretion advised' in general but often change the rating on individual posts, usually bringing it down because the posts don't have any content in them to warrant even a viewer discretion warning. I've also marked several posts as 18+, not because they contain explicit content but because I felt that they weren't appropriate for anyone under 18 to read or because I don't want to speak with anyone under 18 about the topic of the post -- similar to what Makamu says above, I use the 18+ rating as a way of marking the maturity level needed for a discussion more than I use it to mark that the content of the post is explicit in some way.