Deer will eat dahlias - however, from what I understand they are not that adventurous in their eating habits, and if they haven't had mom show them that it's tasty, then most won't try something new unless they are really really hungry. I know lots of people who have had deer damage to other plants, but the deer left the dahlias alone, and I know others, that have had their dahlias chomped.
They only critter I have had problems with in the dahlias was a groundhog.
