My rule of thumb is that drawstring pants (or any pants I'd be comfortable sleeping in) don't belong in public unless I'm going to the doctor's office, have to go to the office on a Saturday, or am working out/riding the bus to the gym. For a superlong flight, though, I'd bring a pair and change into them before going to sleep. I just find that I am treated as a professional (and feel like one) when I dress the part.