Generally it's a bad idea to play 1 or 2 letters in the hope of picking up a specific letter to make a great play. The most common letter in Scrabble is E; there are 12 E's in the set out of a total of 100. This means on average that you only have a 1 in 8 chance of picking an E and that the chances of picking any other specific letters are lower.

Near the end of the game, however, fishing becomes more viable. Obviously you won't know precisely what letters your opponent holds. But you can see what letters have been played and take an educated guess at what's left in the bag.