Although the point value of S is one, and the value of  a blank is nothing, it well worth considering their worth to your rack and in particular their usefulness in generating a large move score.

 

Many top players will rarely use an 'S' for less than 30 points during the first half of the game.

Many top players will rarely use a blank for less than 50 points during the first half of the game.

 

In all likelihood both will form the basis of a bonus word (ie 70+ points).

 

There are exceptions : perhaps you are holding 3 esses, or maybe a rack with no vowels, or there is a high scoring opportunity that needs to be blocked before your opponent uses it. In  general (and probably 80% of the time) the 30/50 values should be considered.

 

As the game progresses the point worth will diminish as the board becomes more blocked, and letters are used up. However, the value of 'the last' S or blank is a useful weapon, so may be worth retaining for the end-game.