You triggered a rant from me which mostly I think doesn't actually apply to your question, so I took it out of the comment and posted it to my journal instead: You're about to view content that the journal owner has marked as possibly inappropriate for anyone.

As to your actual question - the only time I've seen people using rating on their DW journals/comms is when the account was being used almost entirely for organizing fanwork (which a lot of people have stopped doing, since AO3 is so much better at it.) Most people who are journaling just handle general ratings more informally through things like cut tags, and if they warn in the tags it's for specific content types.

But if you are going to put the adult content warning on your journal by default, ratings in the tags is actually a really good idea, because tags are visible below the warning cut, so it's the only way for someone to see which entries actually are adult.