I've never tipped housekeeping, nor have I ever known that was what you were supposed to do. I don't think I've ever seen my Dad either, but I'm not sure about that.
How do you go about tipping housekeeping, BTW? Do you just leave money in the room? I rarely if ever see the maids who make up my room. I'm usually out and about during the day.
I do tip the valet guy though, for sure.