I don’t.
Should I?
Yes, for time of donating or receiving blood. Might help in a situation where you need blood urgently but it would take doctors too long to do the whole testing thing.
Somehow, my father was convinced (and therefore I believed him) that I was O-. It wasn’t until I donated blood in high school that I found out I was, in fact, A+. Thanks, dad!
Yes. If I forget, it is on my driver’s license.
I do, though I didn’t know it until I started donating blood, around a decade ago. (A+, I got the highest grade. 
)
O-negative is the universal donor. In an extreme emergency, that can be given without cross matching.
Moderator Action
Since this is more of an informal poll and an opinion thing, let’s move it to IMHO (from GQ).
Yes.
Blood type is an opinion? 
Mine is boring old O+. My father had a rare type.
If you need to find out, just go and donate blood. And remember you need to remind yourself of it twice a year.
I don’t know if donating my blood saved someone else’s life, but it probably saved mine.
Yes, I know my blood type from donating blood and platelets.
Do you know if you are CMV negative?
Was that because he’s O-? Do you know your mother’s blood type? Sorry, this just reminds me of some of the stories my Mom’s told us of being a nurse…
I believe my mom is A+ and my dad is O-.
I’m positive mine is B-positive. 
I know mainly because I’m a regular donor of blood and platelets at my local blood center. And it is on my old dog tags. (And so is my social security number, not a good thing)
Another A+ here.
Yeah. I found out in high school science class and never forgot it. B+.
Whether you need to KNOW it or not is an opinion.
O- blood here.
AB+.
Universal recipient, bitches!
“Do you know your blood type?” is basically an informal poll.
“Should I?” is asking opinions.