There's no question that traction goes down when wet (coefficient of friction about 50-60% of dry) but there's still plenty of traction available and really no reason to change your braking style (other than less pressure) when it gets wet. Your car still brakes well in the wet and so do your bike tires. If you really need to panic stop skidding your rear wheel will be controllable but it isn't going to slow you down much. If you need to stop you still should be using mostly your front brake.