Hy ff171, I assume the two screws are holding the brake drum to the hub, when you tighten up the screws you bring the brake drum up tight against the hub, duplicating what the wheel and wheel nuts would do. So if you are experiencing a rub there may be a burr on the outside of the hub or the inside of the drum, maybe an errant piece of gasket material. If you can turn the wheel by hand in either direction when the screws are tight I wouldn't worry about it too much. Have you adjusted the brakes yet?