My new Madone road bike came with Ultegra everthing, except it had a 105 chain and cassette. (I hate it when manufacturers cheap-out on the parts that don't bear a model name on it). At 2700 miles, I decided to check stretch. Imagine my surprise when the Park chain check tool showed 1% stretch and more! When the gauged dropped thru the chain link, so did my jaw. I checked with a 12" ruler and confirmed the chain was stretched to the point the cassette was probably damaged too.
Living in Southern CA, I never ridden it in the rain and I lube with Prolink just about after each ride (probably 75 miles between applications). I was surprised to find it stretched so much. I wish I had measured it every few months to monitor it.
I replaced the cassette and chain and with SRAM PG1070 cassette and PC 1090 chain, and hope I get more longevity. I'll be measuring more frequently.
I don't have enough data points yet to know if Shimano chains are susceptible to more stretch or perhaps it's the nature of thinner material used in 10spd chains.