I make my top-levels clickable simply because there could be any reason the dropdown either doesn't work, or the user has a difficult time selecting it (I try to fix this with some JS but I can't guarantee the user has excellent control of a pointing device, or isn't using a touch screen).
Having the top-level be a link to some page offering any sub categories is pretty safe.
Though according to some UX ARIA standards I was reading recently, which suggests what users should be building "widgets" like accordions and tab panels to do, the top-levels are only controls and hitting ENTER would "open" them. Uh, this os the page I was reading.
But I'm uncomfortable with that page's recommendations for a few reasons, one of them being that ARIA is not available to sighted keyboarders and not all AT (esp not old AT). So the last mega-menu I did, each Tab moved forward one obvious menu item, SHIFT-TAB went back one, and since the dropdown appeared (though not in all browsers! IE7/8 wouldn't show), down/right arrow cycled forward in the submenu items, while up/left arrow cycled backwards. Another Tab brought you to the next main-menu item or, at the end, out of the menu entirely and you'd continue tabbing through the rest of the page like normal.