Noting the fact that an animal is 66% het can be a helpful flag to a potential buyer who wants to avoid the gene as much as possible. I took a long time sorting through ads for my last couple future breeder hognoses to avoid all the possible hets floating around from the last couple generations of moving fast and breaking things with that species. Yeah, not every hognose needs to be axanthic, folks.
To answer the original question, it is reasonable in an ad to list 66% het and then also note the number of offspring that did not confirm the het ("66% het, but all 24 offspring with a visual yielded no visuals" or something to that effect). My own interpretation is that the '66% het' designation is based on the parentage, and not on failures to confirm the het (because that's what '66% het' means -- it is a claim about the parentage). If someone sold me a snake that they called '50% het' and I found out that that snake was produced by breeding 2 hets together, I would feel mislead.