Bicycle Tutor

My bike repair shop says that chain stretch should be measured with the chain under 'medium' tension.
I'm not sure how you would do this using the '12-inch ruler' method described in the video on this site. Does the chain need to be under tension when measuring?
My gut tells me the bike shop may be saying this to simulate the chain under stress/in heavy use, but their methods are leading me to a LOT of chain/cassette replacements in the last year. I do ride every day, in all kinds of weather (rain, sun, etc), but I clean and lube the chain each month. (I ride about 5K per year).
thanks.

The chain needs to be held tight enough that each joint is held solid and straight, but not under any load. I just push lightly with my hand on a pedal while I'm measuring. If there's no tension on it, each link could have a slight bend which would mess up your reading. But chains don't "stretch", so I don't think you'd get a different reading under light or heavy tension.
I usually let the chain go a little longer than the normal recommendation, but I also change chain and cassette at the same time routinely. I'm probably putting more wear on my front chainrings, but it hasn't been a problem. Changing chain and cassette more than twice a year seems a little excessive. But it does vary a lot by how much you ride, conditions, how forceful you are, etc.