I very much agree with MercedMike --- a mention by name is worth even more than an additional tip, unless maybe you like throwing big bills around.
That said, there have been a few times when I've tipped over the automatic. But my preference is to tip my cabin steward at the START of the cruise. Since I'm often with another woman, it's not unusual for us to request extra hangers; and when they are delivered, I hand over $10 or $20 WITH THE COMMENT that I don't generally tip extra, but I know how busy they all are on embarkation day....
I have never had anything less than excellent service. Sometimes extraordinary service, like the steward who not only refreshed my ice frequently, but made sure I had a couple of lemon wedges on the tray.