Sorry if this info is already outdated, I started writing this ten hours ago when there was one reply, and then realized I should be outside shoveling from last night's storm, and then shovel roofs that are about to cave, and then plow, and plow, and plow the worst sloppiest snow I've seen, four hours of plowing for four driveways, and then more shoveling...
So... I've been studying this for a while, and I still am.
There will be a few schools of thought, and maybe there should be.
Guitars are not all alike.
I used to ship guitars everywhere, no longer going to happen with the CITES fascism, all you need to do is apply for a permit with the USDA, then Fish and Wildlife is if those guys hadn't done anything but disasters around here, like giving us free wolf packs, but I digress as usual...
When you ship a guitar, you want to get there in one piece, it's just best for everyone, and ultimately, the guitar. Some guitars are more important than people (my own silly opinion), but like my 1973 Guild D35, it's still around, I don't know if anybody that owned it is still around, and frankly, it doesn't really matter. What I'm saying is this guitar might have already outlived a lot of people that were around it, but the guitar remains, and I hope that in twenty years, it is still being played by someone, when I'm no longer around. The guitar will still matter, but I may have returned to matter.
The other day, there was a sad feeBay ad of a beautiful but broken Guild Archtop, the back just about busted clean off from an impact to the endpin, and the ad said "no case, but this time it will be packed in a way that assures no further damage to the guitar".
If you're buying a guitar, you should worry about what the shipper is doing... Coach him if you have to, but if it's like my recent experience, I told the guy how to pack it, and I thought we agreed on Priority Mail and I said I'll chip in over the free shipping in his ad. Next thing I know, he says the clerk at Staples told him it was better to UPS ground it, and they charged $150 to ship a 20lb box, which he then wanted me to chip in on, I mean like wt? I don't even know if he followed a single of my packing instructions, and like sellers who do weird unexpected things, he wouldn't answer anymore emails.
For this one, I wanted him to slack the strings a full turn from standard pitch, put something under the strings, pad the headstock properly, etc etc.
I have shipped a lot of guitars with amazing luck over the years.
A Strat or Tele, you don't need to slack the strings, I don't think.
A Les Paul or SG, loosen the strings a little.
Of course, if it's summer or winter that throws an additional monkeywrench into things. Winter is probably better. I welcome crazing myself, and that's probably the worst that could happen. Summer heat can soften glue, and that could be trouble with a lot of tension on the strings, I'd avoid it.
I did read up on shipping acoustics before even paying for the guitar, and there are a few schools of thought, from tuned to pitch to totally slack.
Totally slack sounds like a bad idea, strings flapping around, and in some cases, I read stories of bridge pins coming out and floating around the case, along with the strings...
Supposedly, all of the big acoustic makers ship with strings up to pitch, and feel fairly condescending about it, but they are brand new guitars, and if anything happens, UPS/FedEx probably takes the blame.
Proponents of shipping at full tension will also have you believing that the weight of the tuners will shear the headstock without string tension, and maybe this is a thing, so there is two reasons to keep some tension on the strings, but maybe back off a little, especially it is old.
With older guitar with steep head angles and sometimes crappy cases, look at how clear the end of the headstock is from the bottom of the case, it it does not clear the case bottom very well, add an old guitar cleaning cloth folded four time or similar where the neck rests on the case support, add as much material as you need to make sure the end or the headstock can never contact the case bottom.
If it's too close and there's a hard impact, like a standing cases falling in its back, it can snap the headstock off the neck.