Sentence:  `` Speed in painting a picture is valid only when it imparts spontaneity and crispness , but unless the artist has lots of experience so that he can control rapid execution , he would <WSD>do</WSD> well to take these first sketches and soberly reorder their design to achieve a unified composition . 
Meaning of the word marked with <WSD> tags:  act in one's own or everybody's best interest