USE BOTH BRAKES!
Here's the (MSF) logic: Train yourself to use both brakes properly for all stops at all times, so when and if you ever need to do a "quick stop" or "emergency stop" your reflexes (muscle memory) will work properly and you will instinctively apply both brakes properly. If during your "normal riding" you fall into bad habits about favoring one brake or the other, when the time comes that you actually need
to use both brakes you will most likely screw up, your muscle memory will most likely fire off before you have time to think about what you should do and you'll brake poorly and increase your risk of injury.
After food shopping, when you come to a stop in your car, did you every notice that your grocery bags fall forwards? That's because the weight of the vehicle (and all of its contents) transfers forwards and loads up the front suspension. This is important!
As long as we're on the subject about braking properly this is something to be very aware of: The Motorcycle Safety Foundation is acutely aware of the three main physical skills that motorcyclists tend to lack that lead to many unnecessary single vehicle (meaning motorcycle only) accidents. 1. The ability to properly take turns. 2. The ability to properly swerve. 3. The ability to properly stop quickly.
So you ask: How do we stop the bike quickly and in the shortest distance possible? Well as we know weight transfers forwards when braking, so the front brake is going to do most of our stopping. As weight transfers forwards the weight on the rear wheel lightens and increases our chances of locking the rear wheel. When braking we want to use both brakes, there's no point to not using the rear brake and missing out on stopping power, but a locked wheel skids and slides, it's not really braking to its fullest potential. Another thing to be cautious about is grabbing or pouncing on the front brake. While the front brake does have more power, if you apply too much brake pressure on the front brake before the weight of the bike transfers forwards you can stop the front wheel from spinning altogether - thus causing a low side accident where the bike just falls face down, usually to the right because your right arm is pulling so hard on the brake. Skidding is not braking!
The answer is to apply a firm progressive ssssqqqqquuuuueeeeeeeeeeeeezzzzzzze to the front brake lever (the more the weight transfers forward during the stop, the more you can squeeze front brake lever for increased stopping power and not lock the front wheel) while simultaneously applying light to lighter pressure on the rear wheel (light pressure to begin with because it's not too hard to lock the rear wheel and gradually lightening the pressure on the rear brake because as the weight transfers forwards it will become even easier to lock the rear wheel. Remember, skidding is not braking, it increases your stopping distance.)
Think you're going to remember to do all of that when it really counts? Trust me you won't, especially if you get sloppy with your everyday braking and save proper braking for special occasions.
If you grab your front brake and over-brake and stop the front wheel from spinning you only have a spilt second to LET GO and let the wheel spin again to try regain traction and then brake again properly -because after all you wanted to stop right? Holding onto a front heel skid pretty much always results in a "low-side" accident. So NEVER grab the front brake lever and if you accidentally do -let go of it!
If you stomp on your rear brake and over-brake you can easily stop your rear wheel from spinning and start it skidding. This is usually accompanied by a loud screeching from the rear tire rubbing on the road surface and most people's natural tendency is to want to stop that sound to they let go of the rear brake. Letting go of the rear brake once it's locked (on a dry surface) will allow the wheel to spin again and usually it very abruptly regains traction which often results in a "high-side" kind of accent where the bike literally throws you off the bike sideways.
On a clean dry surface the easiest way to control a rear wheel skid is to hold the rear brake down and keep skidding. Continue to brake properly with the front brake and come to a full stop. Keep your head and eyes up and level and keep the handlebars square. This is equivalent to "steering into a skid" in a car (as apposed to steering with a skid in a car which could spin the car in circles). Once you've stopped you can of course let go of the rear brake.
On lose surfaces like gravel, sand, mud, rain, ice, etc. you may (depends on your skills, comfort level and the conditions) have the option of easing off a little bit of pressure on a locked rear wheel. Since the surface of the road is lose the available traction is reduced and the rear wheel will not have as an abrupt and violent snap back in line with the front wheel when it starts spinning again.
Bottom line: Get in the good habit of braking properly (with both brakes) at all times and also take some time every now and then to find a nice safe area and practice doing some quick stops.