Did some more research about this.
Adsense doesn't check for nofollow like Googlebot because its function is different. It (should) also only crawls pages that contain ads.
But adsense bot does respect robots.txt. So if you are blocking pages in it that contains ads you should allow 'Mediapartners-Google' to crawl these, else no ads will be shown on those pages google states. Same for private threads (not visible guests), you can allow adsense with a code then.
In both cases google will give a warning about this when access to pages with ads are blocked. So nofollow doen's matter with adsense, it will crawl these pages.