I doubt you'll be significantly less likely to get checked with a 20" widebody than someone carrying standard 22" rollaboard. The only time I've been asked to size my 22" Travelpro is at the AA departure level at LAX (which has a reputation for being very strict.) I think the point with using legal sized bags is that if you get asked to size it (however unlikely) you pass and don't have to check the bag and potentially pay a fee.