Its always been a tough call and is very different from culture to culture as well . The most assertive F/O I have ever heard of was a few years ago in Dallas , all waiting to go at the hold with a huge CB over the airport. An AA MD82 lined up , assesed the weather on his radar having recieved a take off clearance. A voice came over the radio and said , " tower this is the First Officer of American --- , I am not happy with the Captains decision to take off in this weather but he is electing to go "
This was followed by an uneventfull take off .
But it may have been different I guess.