Whilst many people do, and restaurants often serve them this way like a steak, it's best to always cook any meat burgers until the juices run clear and no pink part of the meat remains. This is because, when meat is minced, any bacteria on the outside of the meat will be dragged through to all parts of the meat in the mincing process. Cooking it rare could mean that harmful bacteria on the inside of the burger is still present. It is much safer if you are mincing the meat yourself.