What an interesting question. Who among us ever thought of that?
Till now, I assumed I used whichever thumb was convenient. But no, I use my right, being right-handed, I guess. Is that how it usually works?
And as interesting as this may be, do you think the more time we spend discussing it could be an indicator of how much we need to, you know, get lives? If so, this poll could be more useful that we initially supposed. (My wife just asked me to take out the kitchen garbage, and I said, "I can't, I'm talking with the forum guys about which thumbs we use to press our spacebars," and she gave me her "I married this guy—why?" look.)