We didn't want to do the prepaid, but in the end we did. Good thing, otherwise our room steward would have never gotten one, since we never saw him (or her..who knew?)
Also, I really didn't like the way they strong armed you into tipping the maitr'd. If they are going to include the grat do the whole 9 yards. I did tip the waitstaff in the casino on the last night since they gave me the best service.