Ground meats should be cooked to a minimum internal temperature of 165°F
. The only time I'd consider deviating from this rule is if I'd ground the meat myself and knew exactly what was in it. And ground chicken or turkey should be cooked to 165° no matter what. The nice thing about using a primal cut like beef chuck is that its fat content will keep the burgers nice and moist, even if you do choose to cook them to 165°F.