You certainly could loosen the strings, but I don’t think it’s necessary from a structural point of view. If you’re asking about it from the preventing-strings-from-breaking point of view, it’s really hard to say! Sometimes strings break because they’ve been under tension for a while and the material has gotten a thin spot or has had all the stress it can handle, which would indicate that you might experience fewer broken strings while you’re away if you tuned them all down. But strings also seem to break when they’ve gotten used to a particular tension and then experience a sudden change, which could mean that if they were tuned down for several months, you’d be likely to have some breakage in the process of tuning them back up. So really, your guess is as good as ours on that front!