I did my own study and coming out with this conclusion. I have 4 sites, 2 of them i did not add to google webmaster tools and 2 of them I did added. Now, it's very obvious that the ones I added to google webmaster tools is ranking very poorly, but other two has been ranked much better. Is there a pattern to this?
What are the sites, what links do they have, how old are the domains, what is the content like? There is no pattern to this as I have sites that make no difference if they are in or not - remember that every site is different and will be looked at by Google exactly the same.
i agree with iNET SEO you couldn't conclude it that easy. there are so many factors affecting search engine ranking. My suggestion is enlarge your knowledge on SEO stuff, you'll see that it is a wide wide sphere.