The occasional player never changes letters, arguing that it is a waste of a turn. However if you are continually scoring 10 points or lower because you have poor tiles then a change would probably be the best option.

There may be various reasons for a change:

You rack is not balanced. You have too many vowels or consonants and cannot score well because of this.

You are in danger of getting stuck with the Q at the end of the game.

The scores are close, the board is blocked and there are many useful letters left in the bag, which may, if you get them, win you the game.