Leave it tuned. A guitar is designed to be tuned up to pitch with the truss rod compensating for the resultant pull. De-tuning could introduce over-compensation and bow the neck back (unless the rod is loosened off as well). Why introduce that kind of grief? Is it going by air? If so, not sure how bubble-wrap would behave (ever had an un-opened bag of crisps on a plane? - it really likes to puff out). The important thing is to make sure the headstock can't shift. Plenty packing between it and the void at the top of the case, and if necessary, plenty packing between the neck and any void underneath it. In fact, just make sure the whole guitar can't move at all inside the case. I use scrunched up newspaper (plenty of it) with the guitar wrapped in something non-marking (big clear poly bags etc) to protect the finish from the newsprint. Best to pack it, close the lid, pick it up, shake it around a bit (not TOO violently) and then open it up and check for movement. Similarly, make sure the case is well packed (again newspaper) inside the carton. I've done this for several basses and they have managed to get where they're going (internationally) with no ill effects.