I don't dip the navel, the calves are born out in the pasture on nice dry grass. If they were born in an environment I felt was unclean, I would dip the navel. It has to be right away after they are born. They can get navel ill, and it is very difficult to treat in my experience. I bought a young calf that got navel ill, I treated her aggressively and she died anyhow.
edit here ~ I just read Alans post and I should have added that, by rights, it should be done every calf, every time. He is right about prevention. Dip is cheap, and it only takes a second. I was just telling you what I do. Never lost a pasture calf, but that doesn't mean it doesn't happen.