There is never any advantage to tipping the housekeepers - they seem to expect it, so if you don't tip, they try to improve so you will.

Since the crews do change, the above strategy works for a whole stay.

Only on rare occasions, mind you, do they deserve the tip anyway.

However, if I request extra towels or something extra, I always tip the person who brings it up. Additionally, ask if you can have their name to call and compliment for their excellent service. They'll tell others, and your service will be better from others, too.