When I book a hotel room part of the service I pay for is the housekeeping. If I vomit on the carpet or cause some unusual mess than I will leave a tip to cover the effort of dealing with something out of the ordinary. Other than that tipping housekepping doesnt enter my contemplation.
When they leave sheets in a pile at the end of the bed and a vacuum cleaner and mop in the corner I might tip a cleaner to come and service my room or choose to do it myself.