Tattoo ban should be against military attire no matter where the tattoo is placed. The Navy is the branch of the military that is known for the sailors getting tattoos. Army warfare as a land military strategy should not allow tattoos on parts of the body including elbows and knees.
If someone is willing to fight for their country does it really matter whether or not they have tattoos? Does it really matter where their tattoos are located? When someone joins the military they pretty much give up their right to freedom and personal choice. They spend 24 hours a day doing what someone else tells them to do. If they want a little ink, let them have it.