The regular Heritage policy will cover your instrument if it makes it into the cabin with you. If it goes underneath the plane, their regular policy doesn't cover it. However, they will insure it under the plane for an additional premium, about $25 or $50, as I recall. I have arranged with them in advance that I can email them from the plane after I board and let them know if I need the additional coverage or not, depending on whether the airline let me carry on the guitar or made me gate check it.
If you are going to fly with it, get to the airport early. The new rules are that they are supposed to let you carry it on if it fits - and they can't make you take it out of the luggage bin to make room for someone else's stuff. Also, always carry it to the gate - don't send it with the checked baggage. If they don't let you carry it on, they will gate check it right there at the airplane door and hand carry it to the luggage compartment. When you land, they will give it to you as you exit the plane, along with the people with baby strollers and car seats. One thing though - I only will chance the guitar going underneath on a non-stop flight. I don't trust the airlines not to lose it if I have to change planes.
Bill, tgo