Should you tip the housekeeper at hotels?

No one used to talk about this. But in the past year, several friends, all of them travelers I trust, have told me that when they stay at hotels, they always leave a few bucks on the nightstand for the hotel housekeeping staff.

News to you? This concept is growing. Call it Tipping Creep, which is the slow introduction of new optional surcharges in the world's service industries. But when to do it, where, and why are still open questions.

I asked one friend, who travels a lot for work, why she does it. "Because someone told me once that you should," is all she could think of. Not surprisingly, her rules are fuzzy: Leave money when she's staying for a few days, but not if she's only there for a night or two. Presumably, tipping in that way might encourage better service over time.

And there you have the two rationales for tipping: Because the staff needs it and because it buys better service.