Should disc brake rotors be removed when packing bike for travel?

I recommend removing the disc rotors when you travel with a bike regardless of the packaging method. I have a hard sided case and have done the cardboard box thing (my fat bike does not fit in my hard sided case). Hard sided cases rely on compression to keep everything in place; compression against the flat side of a rotor is not a good thing. Remove the rotors and pack them on each side of a piece of rigid material that will reinforce them and tape the whole thing up. They can be easily put back on with the proper Torx wrench.

If this seems like overkill, keep in mind TSA WILL unpack your bike. Do not bother closing a cardboard box before getting to the airport, because they will just cut it open. I leave the box open and leave a roll of tape on top for them to seal it with. I pack any loose items in the box with the understanding that they will be removed and likely reinserted in a different spot inside the box/case. That being the case, you want loose items padded or enclosed in something for when they end up someplace you didn't intend them.

I usually make sure to take all the same tools I used to disassemble the bike and pack them as well (usually in a separate padded bag) in the same box/container. I have always favored the center lock rotors because they were easier on and off for travel.

A torque wrench is definitely recommended, but some people develop a "feel" for the correct torque. There is usually blue threadlocker compound on the screws. There are small, cheap simple "flexion" torque keys that do the job quite well. My MTB came with one, actually.

I have flown 6x with my bike (Cyclocross Disc) in a soft padded bike bag and never once had issues with the rotors. Numerous other MTB friends never remove the rotors either and have not had trouble when flying.

Echoing the comment by @Daniel, I'd be more focussed on the dropouts and RD and making sure those are supported.

You are way more at risk stripping / losing screws than bending a disc. The wheels will be double padded by the wheel bag and the bike bag.

The thing ive found an issue isn't the rotors but actually the headset/stem (especially an integrated one piece setup) being the issue. My friend has a Specialized Venge Vias (disc) and he obviously had to remove the headset to turn the bars to fit into the rigid bike case. Removing the bar/stem means that there's no spare cable/hose so he had to have the brakes bled then re cabled when he arrived there and the same when he got back as they dont like being taken apart. an expensive trip just to take a disc braked bike.

A few years late, but I thought I'd share my two cents - I recently flew with my mountain bike in an EVOC bike bag, and my front rotor did actually get bent out of shape beyond repair (according the local bike shop). So, I would recommend always removing the rotors.