Originally Posted by Deus Machina
Yes, loosen the strings. I've heard tales of guitars that go into a garage in tune in the fall, come back after winter with a cracked nut or the neck mounts pulling free. Or just snapping the springs when they contract from the cold. But I only have my cheap electric, in Florida weather. The clearcoat didn't like that. It's now a project of mine.
Otherwise, I'd suggest selling it local to make it quick and easy. Or insure it for twice what it's worth, and jump on them if anything happens.
I had a truss rod go on me because I left a guitar in the garage for a few months with a heavy gauge on it.