I would downtune a guitar that's sitting in its case for long periods. I do that at my house. I do same when I ship guitars.
I agree with txbumper that whether it really matters or not depends on whether the conditions are correct. Of course, when you ship, you try to insulate the guitar with the packaging, but that won't stop changes from extreme weather and a multi-day trip from upsetting a stable guitar. I know plenty of folks don't downtune, but I figure the guitar is less likely to suffer harm if it *is* dropped when the neck stress is mostly off. Same reason I pull the endpin.
I do have a few guitars in the house that barely get played. 6 months in a case wouldn't be unusual. Conditions probably don't warrant downtuning, technically, in my house, but it just seems prudent to me. Plus, if we have an icestorm in winter which knocks out power/heat for a time, I don't have to worry much about the guitars.