Each airline seems to have a different set of rules . . . and depending on who you talk to, the rules are different. When ariline folks tell you over phone the rules, get their name!
Years ago flew tandem without boxing it several times; took off pedals,turned the bars, left air in tires, panniers on rear rack with helmets, etc. to protect back wheel.
Also shipped it in tandem (cardboard) box . . . you guessed it,they didn't handle it as well and they bashed up the rea wheel . . . they did pay for new rim/spokes.
If they can see what it is, seems tandem got better treatment.
However, nowadays, with S&S and hard cases for the tandem, it is easier. Although have heard inspectors (Homeland 'In'Security?) have opened up the cases, could not get things back in proper place and messed up the bike/paintjob.