I can only speak on Army ROTC but I am fairly certain that Navy and AF are similar. If you are in college, grades do come first. Branching in the Army depends heavily on a formula that gives a LOT of weight on grades but ALSO places weight on your physical fitness tests, military science participation, AND EC's. For example, my DS participates in football game security, and has taken on a leadership role in that. He also on the color guard. These duties are not mandatory and are thus EC's that give additional points to his OML. Grades come first, but the EC's do help build the score. That score will determine his odds of receiving his first choice of post commission branching (Aviation, infantry etc.)
I hope that helps. I am sure someone else will chime in on Navy and AF.