Dr. Banjo writes:
Ask Dr. Banjo #26 addresses a number of issues about flying with your banjo, but here's some more info:
A non-pro player can include his/her banjo to homeowners insurance, and in many instances it's covered for travel damage or loss. Pro players would need a significantly more expensive type of instrument insurance for appropriate coverage. But of course protection of the instrument is the real goal...
Because there are a fair number of horror stories about banjos damaged in plane travel, some people think a special case is necessary to travel with a valuable banjo. The following is the best of my understanding, which I offer not as "ultimate truth", but informed by a lot of experience:
If you'll be carrying the banjo on, there's no special need for a super strong case, in fact a gig bag can be better since it's more compact and lighter. Just cause you're on a plane doesn't mean it's in any special danger if you just carry it on and off.
Putting the banjo in baggage (as I almost always do, with my $700 Gary Price flight case), is not a typical option with a non-flight case... BUT if the banjo is packed properly, it's extremely unlikely anything bad can happen to it. The most likely problem to happen is that the headstock can get snapped off around the nut... not due to altitude, pressure, temperature, but just plain rough handling, coupled with a case that doesn't fully and firmly support the headstock both above and below. So with bubble wrap or hand towel or other cloth you can wrap the headstock to fully and firmly fill all the available space, and thus support it on all sides. Even some pro flight cases are not designed with this protection built-in, so there are horror stories about bad things happening even in flight cases.
The other precaution would be to roll a thin towel pretty tight like a long tube, and tuck it around the rim, tightly filling the gap from the rim to the inside of the round part of the case. This removes the chance that if the case takes a lateral hit, the resonator would take the brunt... instead it gets transferred to the thick laminated wood rim, which can withstand about anything.
For putting in baggage, loosening the strings makes good sense as it removes tension from the neck, which is theoretically more likely to break if under tension. It would take a mighty big hit to do that, though... Think of what it would take to fracture a baseball bat packed in a hard case!
Mostly, the best advice I have when traveling with a banjo is -- try to play it a bunch, wherever you take it! When retightening the strings, check that the bridge is in the right place, so it plays as in-tune as possible, and then ... play it pretty!