Cruiseguy... you will have a room steward/ess AND and an assistant steward/ess along with the butler. Think of it this way, the butler is there to take care of you. The other two are there to take care of your suite.
In the past we have brought "real" thank you cards for tipping the cabin staff because we felt the standard envelopes a bit impersonal. We put their tips (along with a small gift, like boxed chocolates) out the last night. You might not even see the assistant steward, so be sure to ask if there is one and what his/her name is... that way you can leave something specifically for that person. If you don't, I believe the cabin steward gives a portion of his/her tip to the assistant.