That would be up to Google. They still know your IP address, general location, browser, OS, etc. and can use that to "personalize" your search results.

That can still be 5 different people in one house. Or several hundred in a neighborhood.

Cookies can solve that. If that's how it works is another question.

And use pws=0.

Or use NoCrap Google Search Bar (and Firefox) to add pws=0 automatically: http://userscripts.org/scripts/show/120144

[disclaimer: I'm NoCrap author]

They still cookie you