I've seen both. I think part of the differentiation is how much they fear getting hit. Someone who doesn't want to get hit, oddly, is more likely to raise the chin as they instinctively try to keep their head further from harm. As you said, any reasonable training is likely to stop that from happening. I'd think chin-down is more a posture of attack, given an individual who expects he can actually win (so less fear of getting hit).I reckon the chin would go up more often than down. Instinctively.