Do you need to replace the rotors each time you change the disc pads? Maybe. 50/50.
Live near a hill? Hard on brakes? Have prematurely warped rotors? Here's one possible reason.
A nice list of common problems with brakes and their fixes from an English brake company.
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This site has no affiliation with Mercedes Benz/Daimler Chrysler. All contents Copyright 2011 Richard J. Sexton