I don't think I've ever witnessed early boarding in China. This isn't an issue with me because I usually try to board at last call (so I can snag any open seat, including my favorite, which I've never failed to get).

Rationale (in addition to the aforementioned point):

-the 22" roller board craze hasn't taken hold in China, so the overhead bins always seem to have space
-even when they don't, you're bag is far more likely to end up in the closet than the cargo hold
-more often than not, you're going to sit on the ground for 30 minutes anyway... I see no need to make this number 60
-I can usually snag a seat on the last bus; much better than the subway environment on the first two buses