Trauma_Junkie is really ready for Memorial Day!
The thing is that at the hospital we have to test your blood each and every time you may get blood. While you will always remain A, B, O and Rh negative or Positive, over time- especially if you have had transfusions, you can develop other things such as an anti-D, etc- which are smaller components to blood. So knowing your blood type does not save you from a blood draw for typing and matching.
Now, if you are headed to another country, yes, know your type.
Or if you want to be a power donor for the red cross- of course…