As you may or may not know, we've gravitated largely towards unschooling over the years. That means we don't use workbooks. I have a few lying around, and the children are free to pick them up and do them whenever they want, which they occasionally do. I'm not sure if that justifies keeping them around using up valuable shelf space.
A while ago I started using the "three years ahead" rule. As I've mentioned before, when considering education, I don't think about weeks or months. I think about years. After all, if all goes well, you have years in homeschool, and you have the luxury to let things go for a while and in 6th grade you can wake up and realize your child has terrible penmanship and take a few months and, with his agreement that his handwriting is illegible and it might be useful to him if people can read a note he might want to leave for them, help him practice as a 12 year old with mature understanding and the ability to apply himself instead of having tortured his 1st grade, wiggly, wanting-to-play-in-the-dirt-instead self. (Yeah, yeah learning to write incorrectly and it sticks in your brain that way and is hard to correct etc etc. You are of course free to educate your children how you want. That's why homeschooling is awesome. Do it your way!)
So I was remembering from my childhood, how when I was in 4th grade, first grade workbooks were SO EASY. And when I was in 6th grade, third grade workbooks were SO EASY. And I believe for 12th graders, 8th grade workbooks are pretty easy.
This led me to a somewhat radical conclusion. Why not just wait the three years, and let it be easy? (Or better yet, wait the 10 years.) If I just decided to wait until 6th grade to take a look at the 3rd grade curriculum, I would find, without having done anything at all, that my children already knew the majority, if not all, of it. (This didn't quite apply to math, and for many years, I did not unschool math. Now, however, I have been unschooling math for a year and a half. I'll let you know how it goes.) I'm mainly talking about Language Arts workbooks, reading comprehension workbooks, even Social Studies and Science workbooks. (Although they might not learn the particular information in the Social Studies and Science books, they would have learned other science and other historical facts, about subjects that interested them.)
So this became my rule of thumb. Anything that they would breeze through in three years, I wouldn't bother them with now. Better to spend that time playing or doing whatever it is they are interested in doing. Then, three years later, they can zip through it and it's SO EASY. Or maybe they'll already know it.
You can afford it. You'll still be teaching them in three years. And it's much more efficient and pleasant.