Sometimes I do, but generally I don't. First of all, I'm one of *those* who doesn't ask housekeeping to come in and clean every day. I don't want them to. We make our beds and head out for the day. All I want is clean towels and empty trashes if necessary. I don't change my sheets every day at home--don't need it at a hotel.
I think we've been guilted into (a) thinking it's our responsibility to pick up the slack for employers who don't pay well, and (b) that we ought to help cover the expenses of those who don't make enough money. While I sympathize very much with those in low-paying fields, I can't afford to subsidize them all. We already pay taxes to assist those in need and I would be quite open to increasing what we do on a societal level. But tipping ought to be for excellent service, at the discretion of the giver, and not a Help the World fund for service workers that enables employers to pay less.
Who gets a tip? I'm still fuzzy on the criteria for that. Low paid service workers? What about the groundskeepers, trash emptiers, handypersons etc? Provides personal service? Ok, how about the bathroom cleaning crew in DL? Anyone leaving them a tip? What about all the crew that go in after hours to make DLR fresh and ready for a new day by cleaning gum off sidewalks and getting dirty diapers out of the bushes? The list goes on and on. Although it would be wonderful to be able to tip everyone who 'deserved' it or needed the extra money, it isn't feasible. So...tip if you like, tip if it makes you feel good, tip if you appreciate excellent service, heck--tip if you are having a wonderful expensive vacation and feel kind of guilty that others are slaving away, catering to your hedonism-- but DON'T tip because you feel pressured by what others think of you if you don't.
ETA: When I tip, I tip well. I've never noticed an iota of difference in the service I received.