On bigger aircraft I've never had a problem carrying on. There are two approaches, use a gig bag and refuse to put it below or use the smallest hard case you can and be prepared to gate check it as a last resort. Being very nice and smiley with the gate people and flight attendants helps a lot too. I even have pretty good luck with flight attendants letting me use the business class closets.
On the commuter class stuff, though, it'll be going in the hold in all likelyhood. I would never allow an instrument in a gig bag to go in the hold. So, bring your hard case, but definitely gate check it. The guys loading it on the plane will probably be gentle enough, but crazy things happen to luggage between the terminal and the plane (like getting run over).