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

Ranking more players gives you additional control over the players you draft, but most managers typically rank the first 25 or so players. Any players left unranked are added to the end of your list according to the Yahoo! Sports default rankings. So, if you rank 25 players, player 26 is the best player left based on the default rankings, player 27 the second best, etc.

If there's a player you don't want to draft under any circumstances, you can highlight the player's name and click the left arrow to move him to the Exclude Players list.

Excluded players are not drafted under any circumstances.