The answer depends a lot on where you are staying and how good a job housekeeping does. After the first time they clean (and did a good job) I tip a little higher and tell them I will tip them again on my last night of stay.
That way they keep doing a good job in anticipation of the good tip they will get at the end. I reward them at the end with a tip commensurate with the overall quality of the service I received.
I just don't tip to tip. I reward good service.
Peace and Guidance