It's hard to know where to start Each of the above posters is right and wrong...at the same time. gamby is right that it is best to brake hard before the corner but stuff happens and you occasionally have to make corrections in the corner. hamster is right that the front brake provides most of the deceleration but using only the front brake in most situations decreases your deceleration ability by about 20%. And Homebrew01 is wrong about you being less likely to skid if you use the front brake in dry conditions.

Let's start with braking in a straight line. hamster is correct that many people will tell you to use the front brake only. They would be wrong because they are misinterpreting the physics. In the physics of braking, the maximum amount of deceleration that you can possibly achieve on a bicycle is at the point where the bicycle is just about to spin around the front hub and smash your face into the ground. In other words, you are doing a nose wheelie and on the edge of disaster. Just because this is the maximum deceleration you can achieve, it's not the most desirable for obvious reasons.

Up to the point where the rear wheel leaves the ground, i.e. the rear wheel locks up and starts to skid, the rear wheel is contributing to the deceleration of the bike during braking. How much it contributes is dependent on a number of factors including how hard you are squeezing the brakes and, perhaps more importantly, your body position. If you are seated in a 'normal' position, the bike can develop about 0.5g of deceleration before throwing the rider over the bars. If you move your body back and down just a little (a couple of inches back and an inch down makes a huge difference), you can up that deceleration to 0.9g or roughly double the deceleration. Road bike riders tend not to do the body shift but satch a mountain biker brake and you'll see this body shift all the time. All of the gains in deceleration come from a center of gravity shift to put more force on the rear wheel which results in resistance to flipping the rider over the bars.

In a corner, things start to get trickier because you have different forces acting on the bike/rider system. You have a part of the system that wants to continue in a straight line of the corner and part that is pulling the bike around the corner (the tires). The mass of the bike and rider wanting to go off the corner in a straight line is balanced by the force of the tires pulling the bike around the corner. If you slam on the brakes in the middle of the corner, the tires, especially the rear one, can loose traction and the balance is upset. Now the mass of the rider and bike want to go flying off the corner in a straight line...not good!

The rear tire loses traction more readily because the weight of the rider is shifted towards the front of the bike during braking. In a corner it's harder, although not impossible, to shift the center of gravity rearward to maintain traction on the rear wheel. I shift my weight rearward on corners but I still have to be careful when applying the brakes. If the rear wheel does slide, it's not impossible to recover but you have to be very quick and know which way to shift your weight to reestablish the traction. It's more by 'feel' then by formula. Mountain bike riders do it all the time and a sliding rear wheel in a corner is no big thing.

In the end, I'd say apply brakes as needed but be more cognizant of how your are applying them in corners. You need a more gentle touch and some body english (also known as center of mass shifting) but you can brake in corners. I would also suggest that any road bike rider, get a mountain bike and learn how to ride it off-road. You'll learn more about bicycle handling in one afternoon off-road then you can learn in 30 years on road. Once you've learned how to handle surfaces that vary as widely as off-road surfaces, a hard road surface is simple.