If IE10 installer asked you this question: Do you want websites to track you? () Yes () No I think most people would say No. So, defaulting to No seems like the correct thing to do.

I don't know if that would really hurt Google or Facebook because if you have a google account, you are opted-in to have google or facebook track you.

It might hurt 'lurker' ad networks which track you without you even having any account or relationship with them.