I think it depends how you go about things which often makes the difference between it being pushy and not. Like I said, I wouldn't go in and tell the teacher that my child ought to be on a different level. But I would have gone in, or mentioned at parents evening, that I felt they were managing the existing books very easily and could be more chalenged, giving the teacher the chance to say something like "Yes, but he needs to focus on comprehension in an easier book for now" or "I haven't reassesed his level for a while, I'll take a look"
I don't understand why some parents on here seem so scared of going in to speak to the teacher in case they seem pushy - I'm sure the teacher would rather have a conversation with the parent so they both understand each other, than have the parents fretting secretly. But when I was a child, parents just never came into the school apart from parents evenings, and never asked the teacher what was happening or why, so maybe it goes back to that attitude?