I agree with the Brethren above, the less it's worked, the better the texture of the finished product. I also feel that the tiny air pockets/gaps within the burger helps with providing great flavor. Overworking, packing, or pressing of the ground meat can create a dense burger with little natural juices to be retained within the burger. Sometimes i even shudder when I see someone pressing the burger down on the grill with the spatula causing it to lose all that juicy flavor. But we all have our own individual likes and dislikes.