Dropping the weight is inadvisable, it is usually against the rules in many gyms actually, some gyms I've worked out at actually kicks you out if they catch you doing it. It can lead to serious injury to yourself if it bounces off, which is an obvious. It can cause serious injury to others if you were just so buzzed from a set that you didn't pay attention to your surroundings. One instance of an old time friend of mine several years back, he was doing dumb bell presses with about 90 pounds, didn't notice a guy walking by from behind him and dropped the weight on the guys foot, broke it.
You will injure your lower back if you bend over from your waist up to lower the weights. If you keep your back straight like it should have been during your bicep curls, and then just bend your knees till the weight hits the floor. Don't bend your back though, just your legs.
