There is zero reason to know your blood type, other than curiosity. In an emergency you will get O neg (or possibly O pos if you are male or a female over 50) until your blood is typed which only takes about 10 mins. Even if you say to someone "oh I'm B neg", you will not be given typed blood based on that. Mismatched transfusions go very badly, very quickly, and there is no chance anyone will 'trust' someones word. Certainly the lab will not issue the blood!
Prozac has the best idea though - if you donate you can find out. I recommend enrolling to be a bone marrow donor (doesn't require anything extra from you, the tests are done on the blood you donate) while you are there - we are having major issues currently because we rely on European and Asian donors a lot, and they are unable to travel. I think it's important that we improve our local donor pool.
In general, most bone marrow donations do not involve painful bone marrow biopsies anymore - it's just like donating blood (through a vein in your arm after some stimulating injections). Even if for some reason they do need to collect from the bone, it is done under a light general anaesthetic so is very minimal pain. So I encourage everyone to register as a donor.
If you can't donate blood for whatever reason, you might still be a suitable bone marrow donor -
Register - Strength to Give | Blood Stem Cell Donation . You can enrol with a cheek swab.
I'll get off my soap box now