Leaving the strings at tension will keep the necks on all instruments much, much more stable and yer sound post is a whole lot less likely to move on the big bass. Some people say to tune down a half step to allow for atmospheric flux....
all manufacturers ship tuned more or less to pitch. I've used coroplast for extra protection on the outside of fender type cases and extra foam (or clothes) isn't a bad idea inside the case if the fit isn't super tight already. Doorskin or banana board might work even better. If you're gonna travel with it in the future a Calton case for the slab is a worthwhile investment.
http://www.caltoncases.ab.ca/frame_home.htm
They're much lighter than an anvil type flight case (not much heavier than a fender case) and give the same protection. They're the industry standard fiberglass flight case for acoustic guitars and one will cost almost what your bass did but they are worth it if you travel a lot.
