I think that if the original comment is something frivolous ("Jersey Shore is the worst show ever") then a disagreement is fine ("False, it is the best show evarrrr!")
But when the original comment is more serious, like a religious or political issue (like the one you posted), then I don't think it's appropriate to post a disagreement unless you really think the particular poster would welcome it. I know it's hard to hold your tongue, but I think it's just all around more productive to go send another $10 to Haiti than to try to reason with someone, on facebook.
Having said that, when my mom posts things that I think are stupid, I won't write on her wall but I have no qualms about calling her and telling her why her status makes me concerned that our family suffers from early onset senility.