Those are annoying, because I never catch that I've dreamed about them until it's over. Whenever a friend of mine would get deja vu, she would take it as a signal that she was meant to be here...but I thought it was a signal that I had no free will so I'd get annoyed with the feeling that I dreamed about it before.
Keeping a diary of your dreams is healthy, anyway, in my opinion. In my notebook, I always write on the right-hand panel, and leave the left-hand blank until something happens that reminds me of that dream. Then I leave a note on the left-hand side that "This came true on this day". I re-read the whole thing about once every other month. Sometimes I also write in the left-hand side, "Oh, maybe I can interpret this event that happened in the dream as so-and-so emotion psychology or blah blah spirituality." It can trigger lucid dreams, too, which is a (harmless!) sleep disorder where you carry your waking life reasoning and memory into the dream. Then you can do some pretty cool stuff.