I always leave a tip...I would not want to clean someone else's scummy bathroom habits - I think they well deserve it.

Maybe a higher end hotel has the envelopes to make it more "refined" but we don't frequent the uber high end places, and I've never had someone leave behind the tip (I always write "for housekeeping - thank you" on a note, except on the last day when the note is not necessary.

I usually end up with extra shampoo and towels, and nice smiles in the hallway.