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.