Whom should I send the email to? The Principal Investigator or the postdoc who have done the work? A postdoc has published some works that I am interested in. I have some questions about the work, and I would like to share some idea about the topic. 

Is it considered polite to send the email to the postdoc and CC his PI? If I email the PI only, I might have to wait for a long time. However, the postdoc could not even be in the city. So I am not sure what is the best and politest practice.
If the paper lists a corresponding author, then that is the person to try.
Otherwise, I would just email all the authors and let them decide which one will respond. I don't think it's impolite and I'd rather email too many than too few authors (and risk getting no response).