The size and type of hotel could make a difference. A Hampton Inn will have far fewer Housekeeping employees when compared to a full-service hotel with 500+ rooms.

If I were you, I'd call and speak with the person in charge of Housekeeping after you check in. Ask her or him if there is a preferred method of tipping to ensure it gets to the right person. Doing that also sets up a personal connection with the Housekeeping Supervisor. You won't simply be "Room 301"...you'll be "WrLdTrvLnIndian". That can be helpful if anyone goes wrong or if you have an unusual request at some point.