I never tip housekeeping. I regard it as a 'standardized' service. There are not varying degrees to which my room can be clean, my bed made, my towels replaced and my soaps etc. re-stocked. I do not want or need to bribe them to supply extra water or shampoo (if I need more of the latter I'll simply ask for it at reception) and I do not need a towel shaped like an animal, or indeed anything shaped in any other way than God intended.