Yes, that is my method, too. We stay with something until they learn it.
Schools need grades to keep parents informed of how their child(ren) is doing in the classroom. And to keep the teacher informed, because it would be impossible for every teacher to know how each of her students is doing on a certain topic. And to keep the school administration informed on how the teacher is doing in her classroom, because that's all about the test scores, right? ;)
Anyway, it seems silly to me to assign grades in an elementary homeschool.