Charley, Don, et al,
They might or might not need a neck reset. Depends on how they are taken care of. Have the been exposed to heat? Did they find gramp's guitar in an attic and then sell it to the unwary? Is the unwary passing it on to you?
Certainly, you can make the argument that a lot of the shallow body Gibson, Gretsch and Guild guitars from the '60s and '70s are in need of neck-sets in order to use a Bigsby to it's fullest potential. Sometimes the body of the guitar will flatten out or 'sink in'. Sometimes the neck will warp or come loose.
Every guitar is different. The wood is a variable and the way the guitars have been treated is a variable. I would never buy a Bigsby-equipped axe without checking it out in person, unless the seller was a known quantity and could honestly and correctly represent the guitar as being ready to 'Bigsby-Twang' or not.
That said, I've bought a lot of Guild's that needs neck sets. Every Duane Eddy I've seen or owned needed a neck-set or already had one, including my Blonde DE-500 with the mini buckers that I owned a million years ago. If I had that guitar now, I would find a Gold Harp Tailpiece and have a great, non-trem guitar! Alas, it's gone.
Guys, 'The Bigsby Way' is a Process, a Rite of Passage, etc. 'You gotta go through it to do it!' It's like saying Condolences and Congratulations at the same time! Have fun!