I'm 0- and have given several times with no problems. Last time I gave though, I passed out several times and was treated quite badly by the blood bank nurse. She seemed to think I was lonely and came planning to pass out for the attention. ??? I couldn't believe it. I was told in no uncertain terms not to give again. Her supervisor even called me two days later because she had left a note indicating her concern that I would come in again. Weirdest thing I've ever been accused of.
That said, it took me a year to get off their call list - they called every three months despite my weird reputation.
I agree about the volunteering. I won't step foot in a blood bank again, but I do volunteer to babysit when my church does it's annual blood drive.