No, not necessary. You can wait to replace the rear pads until they warrant replacement due to wear. The vast majority of your braking is done by the front brakes and they can wear several times faster. They also create lots more heat which I think contributes to why there is only noticeable brake dust on the front wheels. The brake pad is made of a friction material (metal, ceramic, etc.) and a binding agent that holds it together. I'm going to guess that it's actually the "melted" or vaporized binding agent that sticks to the front wheels and attracts the dust from the friction material. Otherwise, I'm not sure if the expended friction material would stick. On the rear wheels, the heat build up is so much less that maybe neither the used up binder or friction material can stick to the wheel. When cleaning my rear wheels I've never even noticed a film of dust on them.