Yes it is true. In terms of stopping power, rear brakes do perhaps no more than 20% of the effort. During braking the weight is transfered to the front and the front brakes take most of the load in stopping the car. However the rear brakes play another role: Keeping the car straight during braking. Much also dependes on suspension design and function. On older cars the front end would dive much more than in current cars. That diving action meant that part of the forward motion energy was being absorbed and dissipated by the front suspension, thus assisting the brakes. Today particularly on the volvos, front end does not dive as much and the front brakes take most of the effort. That is why the front rotors are bigger and thicker than the rear ones and the pad area is also bigger then the rear ones. As the brakes basically transform motion energy into heat, that is dissipated to the air, the front ones have a bigger mass and also have actually two layes serarated by internal fins to promote more heat dissipation. On my 850 with 205K miles, rear rotors are still original, but last time the pads were changed, the thickness was approaching the minimun value. The problem with keeping this rotos for too long is metal fatigue. They may crack and results can be disastrous exactly when you need them most- during hard braking.