The last two times my Takamine rode in the cargo bay it arrived with broken tuners despite the heavy travel case. The second time I was actually watching the guy heft luggage from the little trailers onto the cargo belt. The belt was below waist high, which seemed convenient for most of the back packs and suitcases. When he got to my guitar case, he paused to look at the "FRAGILE STICKER", then lifted it up an extra foot or more and dropped it onto the ramp. If they'd have let me out of the plane I would have pasted him a good one, but as it was I had to fume all the way from Denver to Seattle.
Needless to say, the guitar travels in the overhead luggage rack these days. If a question arises, I mention loudly that they broke it last time, and that has so far ended all discussion. Even a jumbo body fits into most of the overhead racks if the case is not heavily padded.
I'm with Rick on this one: Insist on carrying it on, and be prepared to put up with a little flack. The little commuter planes usually have a vertical closet somewhere, or often get loaded by the copilot as you board. So far it's that's been safe practice.
~S~