No kids yet, but growing up, A was the only acceptable grade in core/academic classes (because that's what we were capable of), a B would raise "what happened" type questions and disappointment, and a C was grounds for no screen time until it was fixed (going forward, not retroactively; I know this from experience, heh). But this all only applied to term grades. Individual assignments weren't looked at unless it was required by the teacher. They were more lenient on elective classes (art/shop) and PE, especially since they disagreed with the make-up policy (having to show up and do an extra run for some or all days missed...exactly what a kid out with the flu for a week needs! --_--). I think their approach was pretty reasonable, though I did develop some perfectionist tendencies (or maybe they were innate?)
I do think expecting 100% all the time (or making them think you do) is unreasonable and potentially damaging. However, making it out like grades don't matter is a disservice to children also, in the same way "it doesn't matter what people think" is. Because it does. Grades do matter, but only as long as you're learning something. What your teacher, boss, client, and good friends think of you matters; what your lazy classmates and co-workers think does not.