I think most people tip their room steward at the end of the stay since it is always the same person (unlike room service) but some do it daily and either way is fine.
We tipped the dining room staff at the last supper (ha ha) and the room steward right before we left the room for the last time. We never had room service.
Life's a Beach ... enjoy it on a Cruise!
Carnival Ecstasy 4-06, Carnival Conquest 9-07
Mariner OTS 9-08, Carnival Conquest 2-09
Carnival Conquest 9-09, Carnival Fantasy 1-10
Carnival Triumph 5-11, Mariner OTS 11-11