I ship thousands of instruments of all kinds all the time...
I agree the better your flight case the better your odds. Can't beat a great fitted HARD flight case, probably you don't have one though and would rarely use it, so don't wanna pay for one. fair enough. I also agree keeping it in sight and possession is always the best guarantee.
Most of the time a bass in an average lightweight hard shell is fine to check, usually without oversize charges since they are relatively light compared to regular luggage. BUT I have seen mashed hardshells and broken basses coming out out (RARE, much less than 1% but it happens).
carry-on for most airlines in a gig bag is fine for electric. less fine on a budget airline, but still not impossible Don't tell them you're doing it.. they'll say you must check it. Just turn up at the plane and get on. This is also better for the bass than the freezing pressure drop of the checked compartment.
But yeah man never check a bass in a soft case. never ship an upright without a hard-case - Some idiot will mash your bridge.
For Upright without hard-case.. ugh.. yeah take the train. or drive her down. If not feasible; Buy that hard case you won't regret it then pay the oversized fee and check it it'll be ok and cheaper than buyying it's own seat.
If you check the thing reduce the tension of the strings... not necessarily all the way slack but take it down a couple of whole tones. and allow a few days for your neck to readjust after the pressure changes before deciding to make any truss rod adjustments. It will take a few days to settle before it will stay solidly in tune. if checked they may require you to take it to oversized/fragile baggage area that is a good idea anyways as it "may" get thrown less.
But generally unless it is a ridonculously expensive bass, don't worry it'll probably be fine. I can't count the number of times I have tossed my own bass in its gig bag over a barbed fence OR rammed it into the overhead compartment it still works good

Get on the plane as early in the cue as poss to get more room in the overhead... if no room is left they usually have a closet for crutches and staff luggage etc... so I say don't worry, and don't ship it separately travel with it for best price.