Any more it's probably a crap-shoot.
When I bought our cases the measurement standard that was being used was L X W X H, measured at the edges of the case which would have excluded the molded-in skids. It didn't make sense to me since the ribs added to the girth of the case, but that's how it was calculated back in 2002.
However, I've always made a point of using curb side check-in when we've travelled with our tandem and have my tip money in hand so that the skycap can see it. I've never seen a skycap pull out a tape measure in the past but, then again, it's been a couple years since we've travelled by air and the major carriers have apparently turned checked baggage fees into a revenue stream.