I second this, discs only need machining when warped or when their surface is un-even or damaged as buddha stated. Discs should be machined only when absolutely necessary, as everytime you do it, you're making them thinner, weaker and more prone to warping as a result. In regards to updating Rotors, Cam - I'd start with the fronts only first. In most instances upgrading the fronts alone bring worthwhile improvements to braking. It's a pity that car makers never give brakes the effective ventilation they need, as keeping the rotors cool also brings about improvements such as more resistance to brake fade.