Here are the new U.S. Department of Transportation rules on flying with musical instruments. Sounds like carriers are required to check instruments.
The key passage, per the DOT site:
The rule requires that each U.S. carrier subject to this regulation allow a passenger to carry into the cabin and stow a small musical instrument, such as a violin or a guitar, in a suitable baggage compartment, such as the overhead bin or a closet, or under the seats, in accordance with FAA safety regulations and the carrier’s FAA-approved carry-on baggage program.
Carriers must allow passengers to stow their small musical instruments in an approved stowage area in the cabin if at the time the passenger boards the aircraft such stowage space is available. Under the rule, musical instruments as carry-on items are treated no differently from other carry-on items and the stowage space should be made available for all carry-on items on a “first come, first served” basis. Carriers are not required to give musical instruments priority over other carry-on baggage, therefore passengers traveling with musical instruments may want to buy the pre-boarding option offered by many carriers to ensure that space will be available for them to safely stow their instruments in the cabin.
Maybe we should do like my pal Shelley Doty recommends and carry a copy of this every time we check in for a flight.

