Whether it's butler or chamberbaid (especially with a decent length stay), I try to give a large enough tip to impress and make them know it's not going to be a daily occurrence. Why wait until the end? When they see some sign of reasonable generosity, it can only help in terms of service for the rest of the stay.
If you're going to tip at the end, why not do it sooner to let them know their efforts will be shown proper appreciation?