Preemptive chain replacement to extend cog life makes economic sense if you use expensive cassettes like Dura Ace or Campy Record where the replacement cost far exceeds the chain cost. For more plebeian cassettes like 105, Tiagra or Veloce which aren't that much more expensive than a decent chain, running both into the ground together makes more sense. That doesn't mean running both until the wear exceeds 1% and jeopardizes the chainrings. I typically get 7,000 to 8,000 miles on a cassette/chain pair before the chain exceeds 1/16" elongation but before 1/8" and that's when I replace both.