That thums-up=go to gods, thunbs-down=stay here on earth sounds like an after-the-fact rationalization. For one thing, I don't think the Romans would have so strong an association of "up is where the gods are", given Mt.Olympus was almost more over than up, *everyone* (good, bad, or ugly) who dies going to the underworld, now 'with the gods' anyway, Poseidon and some other gods living places that were definately not up, etc. No, I'm not a historian, but I do know how good culture is at making up "just so" stories.
This Straight Dope column talks a bit more about it:
http://www.straightdope.com/classics/a2_450b.html The fact is we don't know for certain what they actually did, but we do pretty much know why have the traditional thumbs-up=live, thumbs-down=die.