I have clamped my carbon bikes by the top tube, but never hard. If I'm wrenching hard on a bike, then I'll clamp the seatpost, if I'm just doing a quick wash, I'll throw it on the top tube and won't even bother really tightening the clamp down.

Common sense goes a long way.

Kind of like torque wrenches...if you're not an idiot, you probably don't really need one.

That said, if I took my bike to a shop and they didn't clamp the bike by the seatpost, there's no way I'd ever take my bike back there.