When you are braking (still in a straight line), you "should" use both brakes to the point of nearly locking up. This is the ideal for cutting braking distances. I, for example, are too bad a rider to do this and concentrate on the front brake and allow the big thumper's engine braking take care of the rear (unless it's a tight turn where you brake from fourth gear, then I also use the rear brake).
When actually turning, it all depends on the turn. Tight turn that a good berm helps square of, sometimes all rear, no front. A rut to run in, usually no brake at all. A flat turn, often no braking.
Nobody can make a blanket statement about how to brake when turning. Look at the pros. They very seldom come a long way with the rear locked, they lock it just as they swing the rear around to turn.