My parents would be cool about it. My mom likes Nevermore, Slayer, Opeth, Nightwish etc. and I got my dad into Dimmu Borgir, Behemoth and Into Eternity. It is odd because if you saw my parents, you wouldn't be able to tell that they like metal.
Actually, when I think of it, a few people in my family including my parents, my closest cousin and my uncle Marc (who is a Metalhead) would agree to the horns in a picture, especially my uncle, considering I already have a picture of him and I with the horns. Rambleeeeeeeee!
My grandparents wouldn't though. Although, I remember years back, my Grandma said she liked Metallica.