Why? I can see no reason to replace a cassette that doesn't need it. A worn chain will wear out a new cassette in short order, but I have never heard of a worn cassette affecting the life of a new chain (assuming the cassette isn't worn out and skipping). And that certainly hasn't been my experience, see my chain and cassette usage in post 8 of this thread. The only reason I can see to replace the cassette with every chain replacement is if you wait too long to replace the chain and the worn out chain tears up the cassette.

Jim