The maker of my guitar says do NOT slacken strings. He says the instrument is made to be stressed by the strings and if they are slackened it is less balanced and more susceptable to damage. He does reccommend packing the head carefully with paper or bubble wrap to be dure it doesn't get bumped around inside the case.

I have flown many times with instruments and never had a problem (yet). In fact, they have always arrived in-tune!