Making blood donations a habit is a good habit. It helps reduce the risk of heart/cardio problems. Studies have confirmed it. I dint know about this thing till recently when I donated blood along with a few friends of mine. Have a look at this for a clear explanation.
I learnt two things this time. One is the above said thing. You live healthy by donating blood and the second thing is when you have been asked to donate blood ask if it is a direct donation or a replacement. We know what a donation is. When someone needs O+ve only O+ve donors are called for. But in a replacement the blood from the blood bank is used by the doctors for a patient and the same quantity (in units) needs to be replaced. The quantity of blood, not the group of blood, is the only criteria for replacement. They accept any group for replacement. So dont miss out an opportunity to donate blood. Keep your self fit.