Listen to Wonderspark. He knows what he is talking about.
I've never travelled with my Mac Pro, but I do carry tons of computer and A/V equipment all over three or four times a year. If you insist on checking it, use a case like the Pelican, remove drives beforehand, and insure it to the max. You do NOT want to see what really happens in the bag room and on the ramp.
Oh, and I seriously doubt you'll get a Mac Pro on as a carryon on most aircraft these days, especially during the holidays. If you pack accordingly and have to gate check it, then you really have a problem. My preferred method is put equipment in cases, or even original boxes with extra padding, and send via Fedex or UPS.