I wouldn't censor because a book is too easy. DS can check out board books or chapter books - I don't care. Even if the text of a book is "too easy", I figure that if DS is choosing it he must like the pictures or it appeals to him in some way - so go for it.
I do censor for content. DS has anxiety, and some story plots would be really stressful for him. Certain themes can trigger his anxiety, and he can become a bit obsessive about it. So this summer when he was going through a period of anxiety about being separated from me and his dad (irrationally so), and it was impacting his sleep, his desire to go places, etc, I would censored any books that touched on this. It just wasn't worth reinforcing his fears. Now that he has worked through that, I would be much more willing to let him get a book that dealt with that theme. To me, it is one more instance of you never know what is going on in that family - there could be a really well thought out reason a parent is doing something.