Hi hcgaloi,

When a chain needs to be replaced because it has stretched, you should change the cassette also. Why? Because the teeth of the cogs will have been worn lopsided by the stretched chain and mounting a new chain to a cassette with irregularly worn teeth will cause a new set of problems like decreased shifting performance and easier chain slips. This is why many experienced bike mechanics will tell you that cassettes should be considered as "consumables" the same as the chain.

One way to delay replacing the cassette is to replace the chain a little early. Since chains are usually cheaper than a comparable-quality cassette, you save money in the long run. This may be what your LBS is thinking. So the mechanic's advice might be correct.

Kind regards, RoadLight