How do you feel about tipping the person that cleans your hotel room?

It is something that you always do? Is there anyone who doesn’t tip in a hotel room? Does it depend on whether you’ve had good service? Do you feel like cleaning is considered ‘part of their job’ and they shouldn’t need an additional tip? Has anyone ever had the cleaning person leave a note for you asking what they did wrong if you haven’t left a tip after Day 1? How much do you leave if you do tip? Do you leave a tip after each day or at the end of the vacation?