Put me down in the "Let your child be your guide" camp, too. I'd say that if he has already demonstrated his ability to read and understand the material, then there's no need to make him keep demonstrating it every day. Even if they're a bit above his reading level, he'll end up learning over time, because he'll begin to grasp the meaning of words through context. And more importantly, this is reading for pleasure, so it's not like he has to pass a comprehension quiz at the end.
So, as long as he's enjoying himself, have at it.
DD6 asked to explore the adult section at the public library, and so we took a trip this weekend. First she wanted to pick out a book about space, so after I showed her how to look things up in the computer and find the right section, she picked out a book that was WAY over her head. I intervened at that point, picked out another whose cover promised to explain the universe in plain terms, and had her read the first couple of paragraphs to me. She mangled a few words, but did reasonably well, and the only thing I said was, "How do you feel about that?" She felt good about it, so we checked it out.
I'm not sure how it's going to go, but I figure it'll go one of two ways, because few things are ever middle ground with this kid. She'll come to us crying and/or calling herself stupid for not being able to understand it, at which point we'll give her a big hug, remind her she's only 6, how far ahead of her peers she is, and that there are adults who wouldn't be able to understand that book, either, and she'll get there in time, so be patient. Or, she'll race through it, and we'll have a hard time slowing her down at the next trip to the library.
Either way, it'll be good for her.
Despite her feelings of readiness for adult-level books, I still read to her every night, for no other reason than because she loves it. I enjoy it, too, but I'm pretty good with the voices. I'm particularly proud of my Professor McGonagall these days.
