I've shipped and received both with and without cases, and no trouble here. I detune the instrument about 1/2 step, wrap with packing paper (always feared even the remote te possibility of bubble wrap reacting with finish), pad control/switch areas, and then bubble wrap the whole thing. I'll use bubble wrap or peanuts to fill out the remainder of the box, and also do like Bill and reinforce the box with extra cardboard.
With HSCs, I'll wrap and pad (bubble wrap) the bass both inside and out of the case, and still reinforce the box to protect the case from damage.
I think the only time I did some double-boxing was with an Ampeg scroll - no HSC, and feared damage to the big headstock and/or neck. That was State-side, though. Doubt I could have pulled that off with the strict dimensional limits now in place for International/EMS shipments. Barely squeaked in a box nearly the exact size of the HSC when I shipped a bass to S. Korea last year.