I have had a similar question once, where I asked the same about nofollow:
http://www.seomoz.org/q/will-a-mega-menu-on-all-pages-dilute-the-link-juice
In the thread the following was posted:
Should I consider setting "nofollow" for some of the links on other pages than the homepage?
Definitely not. When you nofollow a link, you do not save any link juice. You stop the link juice of flowing to the link but it does not flow to other links on your page.
Best regards,
Rasmus