I don't worry about "teaching them too much" and if that causes problems for my kids in school. We have books on a huge range of topics at home for our kids and really just let them go through whatever they want. DS found some of the engineering and science-y kits himself, and we just let him do what ever interests him. Sometimes it is just throwing various paper airplanes and sometimes he wants to do engineering stuff.
But, I don't worry if he goes in with "too much knowledge". The main issue is not knowing too much but if the school can adjust to the pace your child learns at. You child can still be bored to tears even if he goes in knowing nothing and has to endure repetition that is mind-numbing for your child. That is what we saw in daycare - he learned a lot of material we never went through at home, and yet he was still so bored by the constant repetition that other kids need to learn the same material. That is what kills the joy of learning.
We have him in a school where they are constantly being challenged and yet taking time to work on relative weakness - however, they are a GT school so move faster anyway.
