All of them, here. We've used K-3, 6/5, and part of 7/6. (Dd was in third grade in ps, so we skipped 5/4).

The problem with skipping problems in the problem set that I've heard is that, in the upper grades (5/4 and above), there is content taught thru the problem set. And, even the "review" is designed to be not just repetition but also learning. I'm not sure, frankly, if that is always necessary for every kid, but I'd rather not risk it.

I did let dd start the problem set during our "math time" and then complete it later in the day. She also got a chance to go over her problems the next morning before I looked them over, in order for her to catch any computation problems. This was because she had a few weeks of having 4 or 5 computation errors--"careless errors"--. It seemed to work well for her.