No, you don't need to rank all of the players.

Most managers typically rank the first 25 or so players, enough to cover the first couple of rounds. However, you may rank as many as you like. Any players left unranked are added to the end of your list according to the default rankings. So, if you rank only 25 players, player 26 automatically defaults to the best player remaining based on our default rankings, player 27 is the second best, etc.