I've been to Beijing and Shanghai multiple times (on both legitimate business and get right back on the plane sort of mileage runs), and I have never, ever, had a customs official say a single word to me. Just have a visa, and they don't care.

BTW, the new Beijing Capital airport is quite nice - night and day from the old one.

Shanghai Pudong is nice, but kind of strange in its quietness when the UA flights operate - nearly all shops are closed.

Have fun!

Greg