Mike,
The problem is that there isn't any set policy (and if there is one, it is that it won't go on the plane).
Not only that, but the person that has the final say-so is a line-level employee, and may or may not be aware of corporate policies. Your ability to put your guitar in the cabin may well be regulated by someone that is having a bad day, doesn't like people, hates beards, etc. You get the point-- you are totally at someone's whim.
That person's say will be final, even if they're just being vindictive because they hate guitar players.
I do usually follow the old adage, ask for forgiveness rather than permission, but it just seems too risky to me (to use a gig bag/try to carry on). Especially considering the looks of your bass (i.e. it has many, er, pointy bits).
I never check bags when I'm travelling (unless I'm with my wife, and then I have no choice-- her idea of minimal and mine are somewhat at odds), so I understand your want here, but I'd get the flight case.
Brad
