I know a lot of people that have strong, and very specific opinions about this. But like many things, I don't think there is, or should be, any hard fast rule here. IMO, your downshifting should be determined on what allows you to maintain the best control over your bike at all times, without unnecessarily increasing wear on the bike. To a certain degree, this can vary for everyone, because it is determined by your riding technique and comfort level.
That means that your downshifting technique should vary based on the situation. Coming to a gradual stop at a light will be very different than an emergency stop, or reducing speed while riding aggressively in the twisties.
While approaching a stoplight, I will generally downshift as speed decreases so that I can match my speed with the proper gear, but I won't necessarily let the clutch out. Engine braking provides good control, but I don't use it unless I need it. Brake pads are far cheaper than clutches.
There are a couple of things to consider when it comes to rapidly downshifting through multiple gears at the last minute. First, it's just harder to do. A motorcycle transmission doesn't like downshifting from 5th gear at 10 mph. Second, if an emergency were to pop up, you will likely be in the wrong gear, preventing you from quickly reacting to accelerate away from the danger. Your throttle can be just as important as your brakes in an emergency.