Manage your ads. Define under what condition they will show (visitor from search engine, old post..). Make more money.
Yes. It does not modify, alter, change or whatever synonym you'd like your Adsense / YPN code.
It's been proven by various analytic articles that regular readers come to your site for your content, and don't click ads. On the other hands, new visitors, or people referred to your site by search engines, are likely to click ads.