I don't check the level on every book Jay reads. Frankly, I have better things to do with my time, and have enough trouble just maintaining a list of his reading (in fact I've stopped trying to maintain a list in his school notebook, because he reads so much). I keep track of most of the books he reads on WorldCat, but even that isn't a complete list.
Anyway, I don't check the reading level on everything he reads. I check sometimes when looking for books for him. Apparently, I should start checking more often when I'm searching for books for him. For each topic we're covering in History this year, I made a list of historical fiction books I think he might enjoy. I checked to make sure our library carries them, and wrote the titles & call numbers in his school notebook. Our library doesn't list the levels of the books, so all I have to go on is which section of the library the book is in. That isn't incredibly helpful, as books written for adults could be written at levels low enough for elementary school students.
I do prefer that he doesn't read too many books below his level, because low level books won't challenge him to improve his reading. He is still really into the Goosebumps & American Chillers, so I let him read those despite them being well below what he's capable of reading. Most of his other book choices are 5th-8th grade level. He can read higher than that, but the grade 5-8 range is where he is most comfortable right now.
Well, the list that I made for the Middle Ages has more than a few books that are years below his comfort range. I wish I had realized that before. I'm going to have to look again and see if I can find some more, more challenging, books to add to the list. It's not just about having his reading challenge him. It's also about enjoyment & time. He can easily get through a 3rd grade level book in less than a day, and many books below his comfort level don't hold his attention.
He's actually complaining because the last 3 or 4 books from the list were too easy. Well, I'm off to the library website to look for more books to add to the list. I think I'll also check all the books on the lists for his other History topics. That way, I can make sure the lists are mostly full of books at his comfort level, before he starts the topics.