When Is "Net Grading" Used?

When they feel like it.

It's a judgment call on the part of the grader. Older coins are more likely to be given a straight grade with a net-grade adjustment if they have a minor problem like a small scratch or traces of an old cleaning. That judgment threshold varies from grader to grader and probably for individual graders depending on their mood. A scarce coin with nice eye appeal (other than the minor defect) will likely be a candidate for net-grading while an unattractive piece with a similar level of defect is slapped with a Details grade.

We've all seen coins with significant problems in straight-grade holders; sometimes the only response is "what were they thinking?"