When we reach for it
Checkout, sign-in, submission — the paths where a defect is measured in lost revenue rather than in a ticket. It is also the fastest honest way to catch a whole class of accessibility defect in the pipeline, where fixing it is still cheap.
When we would argue against it
Testing everything end to end. A large, slow, flaky suite gets ignored, and an ignored suite is worse than none because it looks like coverage. Most coverage belongs lower down the pyramid, in tests that run in seconds and fail for one reason.
What it looks like in delivery
A small number of journeys, deterministic fixtures, and traces on failure so a red build is diagnosable rather than rerun. A test that fails randomly is fixed or deleted the same week: a suite people re-run until it goes green has stopped testing anything.