Remember the luggage commercial where the Atlanta Zoo gorillas were moonlighting as Delta baggage handlers? Their offspring work for the TSA now.
While it may be tempting to pack a sleeping bag and shoes in there, that'll entice TSA to pull everything all the way out of the box to look at it. Then they'll carelessly stuff it back in the direction of "in" and slap some tape on the box. It's probably wiser to wrap the frame in bubble wrap or thin foam to save as much paint as possible while not obstructing their view from the top. Remove the derailer and strap it to the inside of the frame to protect it. Partially deflate tires (leave enough air to provide some cushioning but make sure the tires feel soft). Your goal is to let them look inside, grunt "it's a bike alright" and go check somebody else's toothpaste tube. Check another bag, or a larger bag, with all the rest of your gear.