I haven't flown a lot with my fiddle (6 times or so in the last few years) , but have never tuned it down-

Just plan that it will be your carry-on baggage! Don't ever let them check it. There is a case available that fits the official carry-on luggage dimensions, but it requires using a detachable bow case (which is longer than the regulation size). I have a lightweight compact case from Southwest Strings that is almost crescent shaped, so it is fairly easy to get into in overhead bin.

Also, if there is ever any hassle, be sure to use the term violin and not fiddle- it just gets a bit more respect, and emphasize how valuable and fragile it is.

And for Derryman- last time my husband flew w/ his guitar he took it up to gate check it and the flight attendants found a place to stow it inside the plane. On the way back, he sweet-talked his way into getting on with the passengers with small children... and was able to get it in the overhead bin. I don't know how often the stars align for that to happen, but it the airline staff were really accommodating! (Southwest).